Storage bridges have become an ubiquitous part of today's computing ecosystems. The bridges may be external or internal, with the former ones enabling a range of direct-attached storage (DAS) units. These may range from thumb drives using an UFD controller to full-blown RAID towers carrying Infiniband and Thunderbolt links. From a bus-powered DAS viewpoint, Thunderbolt has been restricted to premium devices, but the variants of USB 3.2 have emerged as mass-market high-performance alternatives. USB 3.2 Gen 2x2 enables the highest performance class (up to 20 Gbps) in USB devices without resorting to PCIe tunneling.

The last couple of years have seen many vendors introduce new products in this 20 Gbps-performance class - including portable SSDs and M.2 NVMe SSD enclosures. Host support has also started to look up. The key challenges for enclosures and portable SSDs supporting USB 3.2 Gen 2x2 include handling power consumption and managing thermals. We have been analyzing these aspects (in addition to regular performance numbers) in our reviews of the 20Gbps-performance class PSSDs and enclosures. The review below presents our evaluation report of Akasa's AK-ENU3M2-07 - a USB 3.2 Gen 2x2 enclosure for M.2 NVMe SSDs.

Introduction and Product Impressions

SSD speeds and storage capacity have improved significantly in the last decade, thanks to rapid advancements in flash technology as well as high-speed interfaces / protocols. Starting from 2.5-inch disk drives barely able to saturate the SATA III (6 Gbps) interface in the early 2010s, we now have gumstick- and palm-sized drives with PCIe 4.0 support capable of sustaining more than 7000 MBps (56 Gbps).

These SSDs have also formed the base platform for portable SSDs. Traditionally, such drives have fallen into one of the six categories below, depending on the performance profile and internal components. Recently, we have seen direct flash-to-USB controllers across all but the highest performance tier listed here.

  • 2.5GBps+ class: Thunderbolt SSDs with PCIe 3.0 x4 NVMe drives
  • 2GBps+ class: USB 3.2 Gen 2x2 SSDs with PCIe 3.0 x4 NVMe drives
  • 1GBps+ class: USB 3.2 Gen 2 SSDs with PCIe 3.0 (x4 or x2) NVMe drives
  • 500MBps+ class: USB 3.2 Gen 2 SSDs with SATA drives
  • 400MBps+ class: USB 3.2 Gen 1 SSDs with SATA drives
  • Sub-400MBps+ class: USB 3.2 Gen 1 flash drives with direct flash-to-USB controllers

In addition to portable SSDs, this type of segmentation is also applicable to storage enclosures. Since the mid-2010s, we have seen a regular stream of SSD enclosures hit the market, catering to 2.5", mSATA, and M.2 form-factors.

Akasa is a well-known manufacturer of thermal solutions for computing systems targeting industrial applications as well as home consumers. They have been maintaining a lineup of storage bridge products catering to different market segments since 2013. We reviewed a bunch of their M.2 SATA and NVMe enclosures last year, and came away impressed with their comprehensive lineup addressing different requirements. This review looks at the AK-ENU3M2-07, an aluminum enclosure sporting a USB 3.2 Gen 2x2 20Gbps Type-C upstream interface and a M.2 2230/42/60/80 NVMe downstream port internally.

There are currently two shipping device solutions for USB 3.2 Gen 2x2 - the Silicon Motion SM2320 used in portable SSDs like the Kingston XS2000 is a native UFD controller, while the ASMedia ASM2364 is a bridge solution more suitable for use in enclosures. The AK-ENU3M2-07 uses the latter. The enclosure itself is made of solid aluminum with ridges to aid in heat dissipation. It comes with a single Type-C to Type-C cable rated for 20Gbps operation. A single thermal gap filler is supplied in the package along with a carrying pouch and a user manual.

One of the attractive aspects of the AK-ENU3M2-07 is its tool-free nature. Accessing the internal board for SSD installation is a simple matter of loosening up the two screws on either side of the Type-C port. They are big enough to unscrew without the use of any tools. This allows the bottom panel to be slid out. The board itself is affixed to this panel, and doesn't need to be taken out for any purpose. A plastic tab to hold the M.2 SSD in place is affixed to the 2280 hole by default. Rotating this tab along the notch allows the SSD to be placed in and locked in place. Without the SSD in the picture, further rotation to make the longer arm of the tab parallel to the rear panel allows it to be completely taken out (and affixed to one of the other holes corresponding to 30mm, 40mm, or 60mm SSD lengths). After the installation of the SSD, the thermal gap filler can be placed on top. The gallery below provides pictures of the enclosure as well as the SSD installation steps.

We evaluate M.2 NVMe storage enclosures using the SK hynix Gold P31 1TB NVMe SSD. Since this SSD is used in all the relevant reviews, it makes for an apples-to-apples comparison across different products.

The table below presents a comparative view of the specifications of the different storage bridges and PSSDs presented in this review.

Comparative Storage Bridges Configuration
Downstream Port 1x PCIe 3.0 x4 (M.2 NVMe) 1x PCIe 3.0 x4 (M.2 NVMe)
Upstream Port USB 3.2 Gen 2x2 Type-C USB 3.2 Gen 2x2 Type-C
Bridge Chip ASMedia ASM2364 ASMedia ASM2364
Power Bus Powered Bus Powered
Use Case Tool-free M.2 2230 / 2242 / 2260 / 2280 NVMe SSD enclosure
DIY 2GBps-class, compact, and sturdy portable SSD with a gumstick form-factor
M.2 2230 / 2242 / 2260 / 2280 NVMe SSD enclosure
DIY 2GBps-class, compact, and sturdy portable SSD with a USB flash drive-like form-factor
Physical Dimensions 122 mm x 46 mm x 15 mm 105 mm x 40 mm x 12 mm
Weight 112 grams (without cable / SSD / thermal pad) 60 grams (without cable / SSD ; with thermal pads)
Cable 29 cm USB 3.2 Gen 2x2 Type-C to Type-C 16 cm USB 3.2 Gen 2x2 Type-C to Type-C
16 cmd USB 3.2 Gen 2 Type-C to Type-A
S.M.A.R.T Passthrough Yes Yes
UASP Support Yes Yes
TRIM Passthrough Yes Yes
Hardware Encryption SSD-dependent SSD-dependent
Evaluated Storage SK hynix P31 PCIe 3.0 x4 NVMe SSD
SK hynix 128L 3D TLC
SK hynix P31 PCIe 3.0 x4 NVMe SSD
SK hynix 128L 3D TLC
Price GBP 69 USD 139
Review Link Akasa AK-ENU3M2-07 Review Yottamaster HC2-C3 Review

The key aspect that stands out is how heavy the AK-ENU3M2-07 is, compared to other enclosures using the same bridge chip. While the Akasa enclosure is 112g, the Silverstone MS12 is just 53g, and the Yottamaster HC2 is 60g. This gives the enclosure a higher thermal mass to cool down the SSD inside, and should potentially result in better thermal performance. 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 2021 AnandTech DAS Testbed Configuration
System Intel Quartz Canyon NUC9vXQNX
CPU Intel Xeon E-2286M
Memory ADATA Industrial AD4B3200716G22
32 GB (2x 16GB)
DDR4-3200 ECC @ 22-22-22-52
OS Drive ADATA Industrial IM2P33E8 NVMe 1TB
Secondary Drive SanDisk Extreme PRO M.2 NVMe 3D SSD 1TB
Add-on Card SilverStone Tek SST-ECU06 USB 3.2 Gen 2x2 Type-C Host
OS Windows 10 Enterprise x64 (21H1)
Thanks to ADATA, Intel, and SilverStone Tek for the build components

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 Akasa AK-ENU3M2-07 enclosure in these benchmarks. Prior to providing concluding remarks, we have some observations on the device's power consumption numbers and thermal solution also.

Performance Benchmarks
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  • back2future - Tuesday, August 2, 2022 - link

    Even Thunderbolt 3 enclosures are on a >~$75, ASM4242 (<=64Gb/s) and JHL8440/JHL8540 for TB4 seems being discussed (, with even TB5 data (80Gb/s) given to public 2021, ~one year ago).
    Are there (external consumer) Usb 3.2 hubs available?
  • abufrejoval - Monday, August 8, 2022 - link

    My feeling is that in terms of ease of handling NVMe has just been an enormous stepback vs. SATA SSDs.

    I love how I could just play around with them like in the good old floppy days, put them here, put them there, move them from system to system, have them be a boot drive here, back them up via another one there, join them into a JBOD/RAID0 a little later in life etc. I've always hated storage being tied to a system, something rather unnatural in early computers and something that IMHO Microsoft mostly pushed to fight pirating in pre-permanently-online times.

    Tray-less hot-swap drive bay caddies for where the CD/DVD drives used to go in notebooks extended that enormous flexibility there, too. Sometimes I'd even move a complex long-running application setup (e.g. Ansible) to a different system for speed and then pop it back into the low-noise one where it was supposed to run after being all ready.

    Far too often the ability to pop out every other drive when installing a newish OS was the only way to preserve a production environment from the newcomer thrashing man-years to bits.

    U.2 caddies for M.2 almost deliver similar flexibility again, but come at quite a bit of a premium. But that would only really pay off, if Thunderbolt was universally available on notebooks and NUCs for external attachment.

    Because it's not just speed that suffers from USB vs. PCIe, but also compatibility in the form of device naming.

    If you move M.2 NVMe drives from an on-board slot or TB to something USB or vice versa, there is a good chance you'll loose the ability to boot the system.

    And then there is all sorts of renumbering of partitions, automatic UEFI reconfiguration when disks are inserted temporarily, which have nothing to do with USB vs. NVMe but how ancient DOS (disk operating systems) have a hard time dealing with the dynamics enabled by hardware today.

    The Silicon Motion SM2320 in the Data Traveler proves rather well, that near NVMe performance can be put into a size and power envelope that is no longer a compromise between size and performance: it's not quite µ-SDcard in terms of size, but it's a far step from the clunky 1st gen external TB drive enclosures while performance is getting nicely close to native NVMe.

    What we lack is a physical form factor to go along with it. Yes, external fully plugable is nice. Until you boot or depend on it and move the hosting notebook around. Then you'd rather not have it sticking out, yet removable.

    I never considered PCMCIA as being all too big, but these days SO-DIMM slots are getting killed on Ultrabooks for using much less space. Pretty sure you could fit 8TB of storage and 3 Watts of heat dissipation into that form factor today...

    I'm also pretty sure something like the Data Traveller (0.25-1/2TB) could be fit into an Ultrabook compatbile Compact Flash form factor, which Apple would oppose for the obvious reason that their business model highly depends on extorting for non-expandability.

    If NVMe performance and multiple terabytes could be fit into a µ-SDcard form factor already, none of the above would have been written. I'm not convinced that combination can be achieved very soon.

    So as a consumer, I'd have a simple message to the vendors: please get your act together and offer us the highest level of performance and flexibility at the most reasonable price in a form factor that works across the majority of all personal computing devices!

    But those guys just want to make money; more money by eliminating competition and customer choice.
  • lmcd - Thursday, August 18, 2022 - link

    Honestly what are you even talking about? I pull NVMe drives from machines and boot them from USB enclosures with absolutely no issues and vice versa up to a dozen times a day.

    Your SODIMM comments are completely off base. LPDDR5 notebooks are competing in bandwidth with HEDT 4-channel systems from the simple fact that DIMM pins take space. Laptops with SODIMMs give up a lot for SODIMMs, not just space on the board. A laptop with a high-end LPDDR5 configuration can outcompete most consumer desktops in bandwidth.

    I think your form factor comments demo that you're just spitballing. On the one hand, we've only recently got native USB SSD controllers. Those will enable other form factors. The space will ramp up soon enough and in small enough form to fit happily in a USB-C flash drive form factor. For insertion, CF has a PCIe-based version. You could've, you know, looked at the wikipedia page for CF before posting?
  • FGeorge - Tuesday, November 15, 2022 - link

    I have 3 nvme SSDs which I constantly swap in and out of this enclosure, one has Windows 11 to go, the other one has Linux mint, and the third one I have it with Windows 7, for when I feel nostalgic. I boot from all three of them with the same speed as I get from my internal gen 4 4x4 Samsung 980pro. The only difference is when I run CrystalDiskmark, and the transfer speeds show to be slower with the external SSDs, But that doesn't affect my usage, and it really doesn't get much easier than swapping these drives in and out. With a SATA 3 SSD, I had to connect the data, plus the power cable, my computer today doesn't even have a SATA power cable connected from the PSU. So, this comment of yours bashing NVME SSDs over SATA SSDs doesn't make much sense.
  • CalliopeAdrianna - Sunday, August 21, 2022 - link


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