USB has emerged as the mainstream interface of choice for data transfer from computing platforms to external storage devices. Thunderbolt has traditionally been thought of as a high-end alternative. However, USB has made rapid strides in the last decade in terms of supported bandwidth - From a top speed of 5 Gbps in 2010, the ecosystem moved to devices supporting 10 Gbps in 2015. Late last year, we saw the retail availability of 20 Gbps support with USB 3.2 Gen 2x2 on both the host and device sides. Almost a year down the line, how is the ecosystem shaping up in terms of future potential? Do the Gen 2x2 devices currently available in the retail market live up to their billing? What can consumers do to take advantage of the standard without breaking the bank? Western Digital recently launched the SanDisk Extreme PRO Portable SSD v2 with USB 3.2 Gen 2x2 support. A hands-on review of the SSD sets the perfect background for discussing the above aspects.


The USB Implementers Forum (USB-IF) has been hard at work in bringing new features into the world of USB - In the last five years or so, we have seen the emergence of Type-C, and multiple updates to the USB standard itself. USB4 has been making a lot of news recently (thanks to its implementation in Intel's Tiger Lake, as well as the fact that Intel lent its high-performance Thunderbolt 3 specifications for USB4). However, this piece deals with the most recent specification update prior to that - USB 3.2 Gen 2x2.

USB 3.2 Gen 2x2 (or SuperSpeed USB 20Gbps) - A Brief History

In mid-2017, USB-IF announced USB 3.2 to bring 20 Gbps bandwidth support to the Type-C ecosystem. Type-C supports two sets of high-speed differential pairs. Only one set is used for traditional 10 Gbps operation (in USB 3.2 Gen 2), with the other set used for supporting alternate modes such as DisplayPort. USB 3.2 Gen 2x2 allows this set also to be used for data transmission (when the alternate modes are not needed). This doubled the available data bandwidth to 20 Gbps. A year down the road, ASMedia demonstrated a PHY for the 2x2 operation. At MWC 2019, USB-IF made public the branding strategy for the the different USB 3.2 flavors - SuperSpeed USB for USB 3.2 Gen 1 ( 5Gbps ), SuperSpeed USB 10Gbps for USB 3.2 Gen 2, and SuperSpeed USB 20Gbps for USB 3.2 Gen 2x2. This was followed by the announcement and demonstration of the ASMedia ASM3242 Gen 2x2 controller for hosts and the ASMedia ASM2364 Gen 2x2 to PCIe (NVMe) controller for devices at Computex 2019. At the same show, Phison also announced a single-chip USB 3.2 Gen 2x2 / NAND flash controller, the PS2251-17.

Premium motherboards with USB 3.2 Gen 2x2 ports built-in started appearing towards the end of 2019 with the introduction of the AMD TRX40 chipset, followed later on by the Intel Z490 boards. All these boards enabled the feature using the ASM3242 controller. Vendors also launched stand-alone PCIe 3.0 x4 expansion cards to equip older PCs with USB 3.2 Gen 2x2 (SuperSpeed USB 20Gbps) ports. On the client devices front, initial demonstrations were carried out with USB 3.2 Gen 2x2 enclosures equipped with PCIe 3.0 x4 M.2 NVMe SSDs. Some of these enclosures have made it to the retail market. Vendors such as Western Digital and Seagate have also released external SSDs supporting the USB 3.2 Gen 2x2 standard in the last 12 months.


Getting into the USB 3.2 Gen 2x2 Ecosystem

Consumers wanting to get into the USB 3.2 Gen 2x2 ecosystem can opt to build a PC with one of the TRX40 or Z490 boards supporting SuperSpeed USB 20Gbps. Building a new PC is only an option for a few - for an interface standard to take off, consumers need to either get the port in an off-the-shelf PC, or, have PCIe expansion cards that can be fitted into older systems. GIGABYTE was one of the first tier-one vendors announce such a card - the GIGABYTE GC-USB 3.2 GEN 2x2. However, the card is yet to become available for retail purchase.

AbleConn PEX-UB159 and the ORICO PE20-1C

The currently available options for adding a USB 3.2 Gen 2x2 port to systems that don't come with one built in are listed below:

Yottamaster C5 and Silverstone ECU06

All these require a spare PCIe 3.0 x4 expansion slot in the computer. An interesting aspect to note is that the Silverstone ECU06 and the AbleConn PEX-UB159 do not require any external power. For reasons we haven't taken the trouble to analyze, the ORICO and Yottamaster cards require external power supplied via a SATA power connector. This could be relevant in systems that do not have a spare SATA power connector (as described in the next section). The Yottamaster C5 and the ORICO PE20-1C both seem to use the same PCB, with just the branding on the bracket being different. Additionally, the PE20-1C product tag is printed on the C5 PCB. This is enough evidence to infer that they are both sourced from the same factory line using the same PCB design.

On the device side, one of the most economical ways to adopt USB 3.2 Gen 2x2 remains the purchase of an appropriate enclosure and a compatible SSD:

Coupling any of the above with a high-end NVMe SSD like the SK Hynix P31 (1TB / $135) or WD Black SN750 (1TB / $150, or 2TB / $310) should result in a speedy SuperSpeed USB 20Gbps external SSD for around 20¢/GB. Note that the SSDs mentioned here are specifically those that consistently maintain 1.5 GBps+ direct-to-TLC writes without a significant price premium.

The easier way out is to purchase an off-the-shelf external SSD. There are three options currently in the market, specified here in the order of retail availability date:

  • Western Digital's WD_BLACK P50 (500GB @ $134, 1TB @ $232, and 2TB @ $350)
  • Seagate's FireCuda Gaming SSD (500GB @ $200, 1TB @ $271, and 2TB @ $485
  • Western Digital's SanDisk Extreme PRO Portable SSD v2 (2TB @ $380)

The SanDisk Extreme PRO Portable SSD v2 line will also have a 1TB model, but that will not be available in retail until later this year.

Unless one moves to the high-end of the capacity line with the 2TB models, the cost-per-GB metric (above 20¢ per GB) is simply not competitive against the SuperSpeed USB 10Gbps external SSDs that have taken over the market in the last couple of years. At the 2TB capacity point, excellent external SSDs of this type are available at less than 15¢ per GB, while the best that the above three families can offer is 0.175¢ per GB. As adoption increases, the price should go down, but right now, there is no denying that there is a premium. Is the premium worth it? We review selected components from the above list in an attempt to find the answer.

The Test Devices

Western Digital had sampled a review unit of the WD_BLACK P50 (1TB version) earlier this year, but I had placed it at the bottom of my review queue for a couple of reasons. For starters, none of our direct-attached storage testbeds were SuperSpeed USB 20Gbps-capable. The other reason was that the WD_BLACK P50, despite its wide retail launch, appeared to be more of a technology-demonstration product, with the ecosystem for SuperSpeed USB 20Gbps still in its infancy. Last month, Western Digital doubled down on targeting the USB 3.2 Gen 2x2 device market with the launch of the SanDisk Extreme PRO Portable SSD v2. A review sample was also supplied along with the SanDisk Extreme Portable SSD v2, which was analyzed in detail last week.

Having two Gen 2x2 devices in hand, we felt that the standard was gaining market traction. To get started with the review, we reached out to a couple of the aforementioned expansion card manufacturers, and Yottamaster was the first to respond with a retail sample of the C5 expansion card.

The rest of the review details the steps taken to set up an appropriate testbed for evaluation of USB 3.2 Gen 2x2 devices, followed by a discussion of the features and characteristics of the WD_BLACK P50 and the SanDisk Extreme PRO Portable SSD v2. A look at the performance numbers for various workloads - both synthetic and real-world, as well as accelerated playback of access traces - is provided. As customary, we also explore the worst-case consistency for typical DAS workloads, thermal performance and power consumption numbers. In the last section, we provide some concluding remarks while touching upon the outlook for the USB 3.2 Gen 2x2 (SuperSpeed USB 20Gbps) standard.

Testbed Travails
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  • six_tymes - Monday, October 5, 2020 - link

    spot on. thank you for posting truths.
  • vol.2 - Monday, October 5, 2020 - link

    When I read it, it sounded like the issue was with USB 3.X devices operating on a USB4.0 chipset. My assumption, and I don't think it was explicitly addressed, is that USB4.0 would be "full speed" per it's own specs. Of course, it wasn't explicitly addressed (from what I took away from this), so I phrased my comment as a question; "So wait for USB4 devices then?"
  • repoman27 - Monday, October 5, 2020 - link

    You’re reading is pretty much on the money, but the answer to your question is a bit trickier.

    The first USB4 hosts to hit the market will (probably) be Intel Tiger Lake based products which have integrated Thunderbolt 4 and support USB4 40Gbps. The USB4 spec requires backwards compatibility with USB 3.2, including both the Gen 1 (5Gbps) and Gen 2 (10Gbps) PHYs. It does not, however, require USB3 dual-lane operation (Gen 2x2, 20Gbps), and Intel has not included this optional feature in the controller integrated into Tiger Lake CPUs.

    If Intel doesn’t have any plans for integrated USB 3.2 20Gbps, I fail to see how it becomes widespread, unless Apple and AMD both embrace it in their future chipsets. On the other hand, USB 3.2 devices are probably always going to be cheaper than Thunderbolt or USB4 gear. Paying the premium for a USB 3.2 20Gbps device today is somewhat questionable, unless you have a capable host or the performance difference compared to other products when connected to a 10Gbps port is worth it to you.
  • magreen - Monday, October 5, 2020 - link

    Why will USB3.2 20Gbps devices always be cheaper than USB43 gear? It seems likely USB3.2 20 Gbps will be a niche product and without being produced in high volume, street price won't come down. USB4 might be initially expensive, but volume production and competition may bring street prices down to what we see today for USB3.2 Gen 1 5Gbps devices.
  • repoman27 - Monday, October 5, 2020 - link

    Because USB4 is essentially Thunderbolt, but even more complicated. It will always take way more silicon and way more power than USB3 on the same node. Thus it’s never going to be as cheap. Economies of scale can’t solve everything.
  • vol.2 - Thursday, October 8, 2020 - link

    Sounds about right. I don't have a sudden need for USB 3.2 dual lane. It seems like the best case for more parties to forgo support for it altogether and push USB 4.0. On a related note, USB 3.X has always been very buggy and unstable/unreliable for me, so hopefully 4.X fixes some of that.
  • Meteor2 - Monday, October 26, 2020 - link

    Not if you plug a USB4-20Gbps or a USB4-40Gbps SSD into them.
  • YB1064 - Tuesday, October 6, 2020 - link

    Why is the ASM2364 winning in every scenario compared to Thunderbolt, if TB offers higher bandwidth? Am I reading this incorrectly?

    WTF is this godawful nomenclature dumpster fire??? The idiots on the USB standards committee need to be flogged with extreme prejudice.
  • repoman27 - Tuesday, October 6, 2020 - link

    The ASMedia ASM2364 is the PCIe NVMe to USB3 20Gbps bridge chip used by the devices being tested for this review.

    The ASMedia ASM3242 is the USB3 20Gbps host controller that was used for testing these USB3 20Gbps drives.

    The Intel JHL6540 is a Thunderbolt controller which includes an integrated USB3 10Gbps host controller for interoperability with USB3 devices. This was used to test these USB3 drives while connected to a USB3 10Gbps host. The ASM3242 is winning because it supports twice the USB3 signaling rate as the JHL6540.

    When connected to a Thunderbolt 3 device, the JHL6540 supports up to 40 Gbit/s. In some of the tests, the ASM3242 card was actually plugged into a Thunderbolt 3 enclosure and connected to the host PC via the JHL6540.
  • Googer - Tuesday, October 6, 2020 - link

    It's kind of sad to think that USB is now faster than SATA. Will there ever be a SATA 4 for SSD and future bulk storage technologies?

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