Western Digital is introducing a new flagship portable SSD today under the SanDisk Professional brand. The PRO-G40 dual-mode PSSD ticks all the right boxes for prosumers, content creators, and studios looking for a high-performance rugged bus-powered portable storage drive with wide compatibility. 1TB and 2TB versions are available. While the PSSD delivers its maximum performance (close to 3GBps) when connected to Thunderbolt 3 hosts, it can also work as a 1GBps-class drive when connected to a USB 3.2 Gen 2 host port. Western Digital provided us with a 2TB sample of the PRO-G40 to put through our rigorous direct-attached storage evaluation.

Introduction

External bus-powered storage devices have grown both in storage capacity as well as speeds over the last decade. Thanks to rapid advancements in flash technology (including the advent of 3D NAND and NVMe) as well as faster host interfaces (such as Thunderbolt 3 and USB 3.x), we now have palm-sized flash-based storage devices capable of delivering 3GBps+ speeds. While those speeds can be achieved with Thunderbolt 4, mass-market devices have to rely on USB. Within the USB ecosystem, USB 3.2 Gen 2 (10 Gbps) is fast becoming the entry level for thumb drives and portable SSDs.

The initial wave of Thunderbolt 3-based portable SSDs and enclosures offered impressive performance, but market acceptance was limited by their incompatibility with traditional USB ports. Intel's Titan Ridge controllers offered an interesting feature when used in client devices such as peripherals and/or docks. In addition to downstream PCIe lanes, an USB 3.2 Gen 2 host interface was also included. This allowed manufacturers to offer unique dual-mode designs by tweaking the traditional JHL7440 Thunderbolt 3 SSD. The USB 3.2 Gen 2 downstream port is connected to the upstream interface of a USB to NVMe bridge chip. Depending on the host to which the drive is connected (reported by the JHL7440), the PCIe 3.0 x4 lanes of the NVMe SSD are connected to either the JHL7440's downstream PCIe 3.0 x4 lanes or the bridge chip's PCIe 3.0 x4 lanes. The Sabrent ROCKET XTRM-Q was one of the first PSSD lineups to support dual-mode operation based on this architecture.

Western Digital is joining the dual-mode bandwagon today with the SanDisk Professional PRO-G40 Portable SSD. The PSSD uses the WD_BLACK SN750E M.2 2280 drive internally. On the bridge side, the JHL7440 Titan Ridge Thunderbolt 3 controller is augmented by the ASMedia ASM2362 USB 3.2 Gen 2 - NVMe bridge chip. The enclosure itself is quite stylish, and is made of solid aluminum. One side is covered by a silicone base. The drive also includes a LED indicator and a metal-reinforced USB-C port.

These features allow the PRO-G40 to obtain an IP68 rating - complete protection against dust ingress, and no damage resulting from complete and continuous impression under 1m of water. The solid aluminum core allows for drop- (up to 3m) and crush-resistance (up to 4000 lbs.) also, while supporting faster heat dissipation as part of the thermal solution.

The PRO-G40 PSSD comes in a minimal package with the main unit and a short 20cm Thunderbolt 3 passive cable. The length could be a concern for typical desktops, but the use-case for these dual-mode drives mostly involves notebooks or systems that utilize a Thunderbolt dock as part of the workspace.

The drive comes formatted for Macs out of the box, but it is trivial to reformat in one's OS of choice. Our sample was evaluated with the exFAT filesystem on a Windows 10 machine. The 'issues' plaguing all Thunderbolt SSDs on Windows affect the SanDisk Professional PRO-G40 too. Unless write caching is enabled in the volume policies for the Thunderbolt port-connected drive, the write performance doesn't match the expectations / advertised numbers.

Our policy is to evaluate all external storage devices with default OS settings - and this has been for quick removal since we moved to Windows 10 on our new testbed. However, we did evaluate the PRO-G40 drive thrice - once connected to a Thunderbolt 3 port, and another time connected to a USB 3.2 Gen 2x2 port enabled by the ASMedia ASM2342 controller. In our final pass, we repeated the evaluation procedure with the write cache settings modified, as shown in the picture below (default settings on the left, and modified one on the right).

CrystalDiskInfo provides a quick overview of the PSSD as seen by the host system. The interface section is of particular interest - NVM Express over Thunderbolt, and UASP (NVM Express) over USB 3.2 Gen 2x2 Type-C.

Prior to looking at the benchmark numbers, power consumption, and thermal solution effectiveness, a description of the testbed setup and evaluation methodology is provided.

Testbed Setup and Evaluation Methodology

Direct-attached storage devices are evaluated using the Quartz Canyon NUC (essentially, the Xeon / ECC version of the Ghost Canyon NUC) configured with 2x 16GB DDR4-2667 ECC SODIMMs and a PCIe 3.0 x4 NVMe SSD - the IM2P33E8 1TB from ADATA.

The most attractive aspect of the Quartz Canyon NUC is the presence of two PCIe slots (electrically, x16 and x4) for add-in cards. In the absence of a discrete GPU - for which there is no need in a DAS testbed - both slots are available. In fact, we also added a spare SanDisk Extreme PRO M.2 NVMe SSD to the CPU direct-attached M.2 22110 slot in the baseboard in order to avoid DMI bottlenecks when evaluating Thunderbolt 3 devices. This still allows for two add-in cards operating at x8 (x16 electrical) and x4 (x4 electrical). Since the Quartz Canyon NUC doesn't have a native USB 3.2 Gen 2x2 port, Silverstone's SST-ECU06 add-in card was installed in the x4 slot. All non-Thunderbolt devices are tested using the Type-C port enabled by the SST-ECU06.

The specifications of the testbed are summarized in the table below:

The testbed hardware is only one segment of the evaluation. Over the last few years, the typical direct-attached storage workloads for memory cards have also evolved. High bit-rate 4K videos at 60fps have become quite common, and 8K videos are starting to make an appearance. Game install sizes have also grown steadily even in portable game consoles, thanks to high resolution textures and artwork. Keeping these in mind, our evaluation scheme for direct-attached storage devices involves multiple workloads which are described in detail in the corresponding sections.

• Synthetic workloads using CrystalDiskMark and ATTO
• Real-world access traces using PCMark 10's storage benchmark
• Custom robocopy workloads reflective of typical DAS usage
• Sequential write stress test

In the next section, we have an overview of the performance of the SanDisk Professional PRO-G40 2TB PSSD in these benchmarks. Prior to providing concluding remarks, we have some observations on the device's power consumption numbers and thermal solution also.

Synthetic Benchmarks - ATTO and CrystalDiskMark

Benchmarks such as ATTO and CrystalDiskMark help provide a quick look at the performance of the direct-attached storage device. The results translate to the instantaneous performance numbers that consumers can expect for specific workloads, but do not account for changes in behavior when the unit is subject to long-term conditioning and/or thermal throttling. Yet another use of these synthetic benchmarks is the ability to gather information regarding support for specific storage device features that affect performance.

Western Digital claims read and write speeds of 2700 MBps and 1900 MBps respectively, and these are backed up by the ATTO benchmarks provided below. In fact, WD has been quite conservative in their claims, and has actually put in numbers that are more representative of sustained usage for the PRO-G40's use-case. ATTO benchmarking is restricted to a single configuration in terms of queue depth, and is only representative of a small sub-set of real-world workloads. It does allow the visualization of change in transfer rates as the I/O size changes, with optimal performance being reached around 1 MB for a queue depth of 4. The effect of enabling write caching are also evident at higher I/O sizes, with the feature enabling better consistency.

CrystalDiskMark Benchmarks
TOP: SDP PRO-G40 2TBSDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TB	BOTTOM: SDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TBSDP PRO-G40 2TB

The 32GB data span used in our portable SSD reviews fits in nicely with the amount of SLC cache present in the PRO-G40 (as we shall see further down in this review). Read and write numbers are well beyond the advertised values, and the performance scales with the host interface - acting as a 1GBps-class drive with non-Thunderbolt ports, and as a 2.5GBps+ drive when connected to a Thunderbolt 3 host.

AnandTech DAS Suite - Benchmarking for Performance Consistency

Our testing methodology for storage bridges / direct-attached storage units takes into consideration the usual use-case for such devices. The most common usage scenario is transfer of large amounts of photos and videos to and from the unit. Other usage scenarios include the use of the unit as a download or install location for games and importing files directly from it into a multimedia editing program such as Adobe Photoshop. Some users may even opt to boot an OS off an external storage device.

The AnandTech DAS Suite tackles the first use-case. The evaluation involves processing five different workloads:

• AV: Multimedia content with audio and video files totalling 24.03 GB over 1263 files in 109 sub-folders
• Home: Photos and document files totalling 18.86 GB over 7627 files in 382 sub-folders
• BR: Blu-ray folder structure totalling 23.09 GB over 111 files in 10 sub-folders
• ISOs: OS installation files (ISOs) totalling 28.61 GB over 4 files in one folder
• Disk-to-Disk: Addition of 223.32 GB spread over 171 files in 29 sub-folders to the above four workloads (total of 317.91 GB over 9176 files in 535 sub-folders)

Except for the 'Disk-to-Disk' workload, each data set is first placed in a 29GB RAM drive, and a robocopy command is issue to transfer it to the external storage unit (formatted in exFAT for flash-based units, and NTFS for HDD-based units).

robocopy /NP /MIR /NFL /J /NDL /MT:32 $SRC_PATH $DEST_PATH

Upon completion of the transfer (write test), the contents from the unit are read back into the RAM drive (read test) after a 10 second idling interval. This process is repeated three times for each workload. Read and write speeds, as well as the time taken to complete each pass are recorded. Whenever possible, the temperature of the external storage device is recorded during the idling intervals. Bandwidth for each data set is computed as the average of all three passes.

The 'Disk-to-Disk' workload involves a similar process, but with one iteration only. The data is copied to the external unit from the CPU-attached NVMe drive, and then copied back to the internal drive. It does include more amount of continuous data transfer in a single direction, as data that doesn't fit in the RAM drive is also part of the workload set.

AnandTech DAS Suite - Performance Consistency
TOP: SDP PRO-G40 2TBSDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TB	BOTTOM: SDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TBSDP PRO-G40 2TB

The first three sets of writes and reads correspond to the AV suite. A small gap (for the transfer of the video suite from the internal SSD to the RAM drive) is followed by three sets for the Home suite. Another small RAM-drive transfer gap is followed by three sets for the Blu-ray folder. This is followed up with the large-sized ISO files set. Finally, we have the single disk-to-disk transfer set. Readers are encouraged to compare the performance profile of the PRO-G40 under default and write cache on settings. The significant reduction in the completion time for the workloads with the caching turned on is obvious. The temperature of the PSSD is also cooler despite the workloads completing faster. The throttling in either case appears to be due to SLC cache sizing and unrelated to the thermals. The drop in write performance to 400MBps during the disk-to-disk write transfers for the default case with Thunderbolt is particularly worrisome and not seen in any other comparison graph above - but that number is still much better than what we saw in previous off-the-shelf Thunderbolt 3 PSSDs such as the Samsung Portable SSD X5 and the Sabrent ROCKET XTRM-Q. We have not reproduced the graphs for those PSSDs above, as they were recorded with a previous version of our DAS test suite and on a different testbed. However, they are still worth referencing when considering the performance profiles of PSSDs connected to a Thunderbolt host.

PCMark 10 Storage Bench - Real-World Access Traces

There are a number of storage benchmarks that can subject a device to artificial access traces by varying the mix of reads and writes, the access block sizes, and the queue depth / number of outstanding data requests. We saw results from two popular ones - ATTO, and CrystalDiskMark - in a previous section. More serious benchmarks, however, actually replicate access traces from real-world workloads to determine the suitability of a particular device for a particular workload. Real-world access traces may be used for simulating the behavior of computing activities that are limited by storage performance. Examples include booting an operating system or loading a particular game from the disk.

PCMark 10's storage bench (introduced in v2.1.2153) includes four storage benchmarks that use relevant real-world traces from popular applications and common tasks to fully test the performance of the latest modern drives:

• The Full System Drive Benchmark uses a wide-ranging set of real-world traces from popular applications and common tasks to fully test the performance of the fastest modern drives. It involves a total of 204 GB of write traffic.
• The Quick System Drive Benchmark is a shorter test with a smaller set of less demanding real-world traces. It subjects the device to 23 GB of writes.
• The Data Drive Benchmark is designed to test drives that are used for storing files rather than applications. These typically include NAS drives, USB sticks, memory cards, and other external storage devices. The device is subjected to 15 GB of writes.
• The Drive Performance Consistency Test is a long-running and extremely demanding test with a heavy, continuous load for expert users. In-depth reporting shows how the performance of the drive varies under different conditions. This writes more than 23 TB of data to the drive.

Despite the data drive benchmark appearing most suitable for testing direct-attached storage, we opt to run the full system drive benchmark as part of our evaluation flow. Many of us use portable flash drives as boot drives and storage for Steam games. These types of use-cases are addressed only in the full system drive benchmark.

The Full System Drive Benchmark comprises of 23 different traces. For the purpose of presenting results, we classify them under five different categories:

• Boot: Replay of storage access trace recorded while booting Windows 10
• Creative: Replay of storage access traces recorded during the start up and usage of Adobe applications such as Acrobat, After Effects, Illustrator, Premiere Pro, Lightroom, and Photoshop.
• Office: Replay of storage access traces recorded during the usage of Microsoft Office applications such as Excel and Powerpoint.
• Gaming: Replay of storage access traces recorded during the start up of games such as Battlefield V, Call of Duty Black Ops 4, and Overwatch.
• File Transfers: Replay of storage access traces (Write-Only, Read-Write, and Read-Only) recorded during the transfer of data such as ISOs and photographs.

PCMark 10 also generates an overall score, bandwidth, and average latency number for quick comparison of different drives. The sub-sections in the rest of the page reference the access traces specified in the PCMark 10 Technical Guide.

Booting Windows 10

The read-write bandwidth recorded for each drive in the boo access trace is presented below.

Full System Drive Benchmark Bandwidth (MBps)Full System Drive Benchmark Latency (us)Full System Drive Benchmark ScoreExpand All

The overall scores are along expected lines - the Thunderbolt 3 mode with write caching on performs on par with a premium PCIe Gen 3.0 x4 internal SSD, while the other PSSD configurations / host connections follow well behind in terms of both bandwidth and latency.

Miscellaneous Aspects and Concluding Remarks

The performance of the portable SSDs in various real-world access traces as well as synthetic workloads was brought out in the preceding sections. We also looked at the performance consistency for these cases. Power users may also be interested in performance consistency under worst-case conditions, as well as drive power consumption. The latter is also important when used with battery powered devices such as notebooks and smartphones. Pricing is also an important aspect. We analyze each of these in detail below.

Worst-Case Performance Consistency

Flash-based storage devices tend to slow down in unpredictable ways when subject to a large number of small-sized random writes. Many benchmarks use that scheme to pre-condition devices prior to the actual testing in order to get a worst-case representative number. Fortunately, such workloads are uncommon for direct-attached storage devices, where workloads are largely sequential in nature. Use of SLC caching as well as firmware caps to prevent overheating may cause drop in write speeds when a flash-based DAS device is subject to sustained sequential writes.

Our Sequential Writes Performance Consistency Test configures the device as a raw physical disk (after deleting configured volumes). A fio workload is set up to write sequential data to the raw drive with a block size of 128K and iodepth of 32 to cover 90% of the drive capacity. The internal temperature is recorded at either end of the workload, while the instantaneous write data rate and cumulative total write data amount are recorded at 1-second intervals.

CrystalDiskMark Workloads - Power Consumption
TOP: SDP PRO-G40 2TBSDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TB	BOTTOM: SDP PRO-G40 2TB [ASM2342]SDP PRO-G40 2TB [W-Cache On]SDP G-DRIVE 2TBSDP G-DRIVE ArmorLock 2TBKingston XS2000 2TBSamsung T7 Shield 2TBSDP PRO-G40 2TB

In Thunderbolt mode, the drive never enters a low-power state, with power consumption peaking at around 7.2W. Idle power consumption is around 2.5W. In contrast, when connected to a USB 3.2 Gen 2 host, the drive has three distinct idling power levels - around 2.5W immediately after traffic, droppiing down to around 1.2W after 5 minutes of idling, and finally 0.9W after another 15 minutes or so.

Pricing and Competitive Positioning

The SanDisk Professional branding was created as an evolution of G-Technology in order to bring SanDisk's reputation into the prosumer market too. It must be noted that the brand targets customers who value performance, reliability, and consistency over price. In business use-cases, this is entirely justified, as the value of the time spent waiting for data transfers and/or troubleshooting malfunctioning drives can easily add up over the lifespan of the device and end up being a lot more than the initial premium.

The 1TB version comes in at $300, while the 2TB version evaluated today is priced at $450. SanDisk Professional's own G-DRIVE PRO SSD, a Thunderbolt 3-only PSSD, has a street price of $430 for the 2TB version. In that context, a $20 premium for dual-mode operation with wider compatibility and an improved industrial design (subjectively speaking) is worth it. Surprisingly, other non-DIY Thunderbolt 3 SSDs are priced similarly. The only other dual-mode PSSD we are aware of in the market is the Sabrent ROCKET XTRM-Q. While the 1TB version is $80 cheaper at $220, its higher capacity versions seem to have been discontinued. In any case, Sabrent uses QLC for that lineup, and that technology is currently a non-starter for the type of professional use-cases targeted by the PRO-G40.

Final Words

The SanDisk Professional PRO-G40 is a versatile portable SSD that lives up to its billing - it has a sleek industrial design that doesn't sacrifice on thermal performance. The performance itself is top-notch when considered against other Thunderbolt / dual-mode SSDs. The IP68 rating, along with the drop- and crush- resistance is a big plus.

The only caveat is that the performance (at least on hosts running Windows) is heavily influenced by the write caching setting. With the default options, the performance drops off sharply after the SLC cache runs out. 650 MBps is not worthy of even a USB 3.2 Gen 2 PSSD. In addition, a SLC cache of around 32GB on an empty 2TB drive seems a bit low for the intended use-cases. That said, enabling the write caching helps restore performance to acceptable levels - close to Western Digital's advertised claims. This issue is not unique to the PRO-G40, but is seen across all Thunderbolt PSSDs.

The absence of USB 3.2 Gen 2x2 support, hardware encryption, and 4TB+ capacities are puzzling. Hopefully, these are features that Western Digital will consider in future iterations of the product line. USB4 PSSDs that can deliver the best possible performance based on the host interface used are definitely the holy grail in the first context. While the dual-mode PRO-G40 is a step in the right direction, it still doesn't provided 20Gbps speeds when connected to USB 3.2 Gen 2x2 hosts.

Our recommendation for overall performance and feature set continues to be the SanDisk Extreme PRO Portable SSD v2. It can't quite reach the peak performance numbers or match the ruggedness of the PRO-G40, but has the best performance consistency we have seen in any PSSD (along with hardware encryption support). That said, for the use-cases targeted by the PRO-G40, its higher peak speeds, wider compatibility, and ingress protection can definitely tilt the balance. Western Digital has PSSDs tuned for different market segments, and the SanDisk Professional PRO-G40 successfully fits into its own niche.