By now, we should all be thoroughly confused with USB 3.1 Gen 2, 3.2 Gen 2×2, USB4, Thunderbolt 3 and 4. The fact that they can all be connected using a Thunderbolt 4 cable to a USB-C port on your Mac is of absolutely no help either. Given the lack of support for USB 3.x SSDs at speeds more than about 1 GB/s, today’s quest is for something faster, with proper Trim and SMART health indicator support.

Before we can go shopping, let’s clarify some terms we’re likely to meet.

NVMe and PCIe

NVM Express, universally abbreviated to NVMe, provides the device interface for faster SSDs, in this case generally those of M.2 physical format or ‘form factor’. It’s preferred over the slower Serial ATA or SATA, which isn’t well-supported by macOS. Support for NVMe was introduced in OS X Yosemite 10.10.3, and its hardware interface first came in 2016 Mac notebooks.

PCI Express, normally shortened to PCIe, is a high speed expansion bus, providing the link between SSD and Mac. This comes in versions up to 7.0 and multiple lanes, usually given in its x number, from x1 to x16. A typical minimum requirement is PCIe 3.0 x4, providing four lanes each at 8 GT/s for a total of just under 4 GB/s throughput, although we’re not going to see anything on the higher side of 3 GB/s in practice.

Thunderbolt

PCIe is one of the three components in Thunderbolt, alongside DisplayPort for displays, and power for external devices. Thunderbolt 3 supports PCIe 3.0 x4 and (as a fallback) USB 3.1 Gen 2 for data connections. Thunderbolt 4 extends that with USB4 support.

Intel Thunderbolt controllers can support one or two ports with a single PCIe 3.0 x4 link, or a low power single port with a PCIe x2 link. The first of those is relevant to Intel Macs, whose built-in Thunderbolt controllers each have to support two ports using a single PCIe x4 link, so transferring data on both ports at once can result in reduced transfer rates. Apple silicon Macs effectively have one controller per port, ensuring that they can sustain high transfer speeds throughout.

Intel has also supplied a Thunderbolt controller intended for low power use: its JHL6240 chip can only deliver a PCIe 3.0 x2 link, two lanes of the four expected in Thunderbolt 3, which explains why some Thunderbolt 3 peripherals only manage half the expected transfer rates.

To recap, by this stage we know we’re looking for NVMe with PCIe 3.0 x4 as a minimum, over Thunderbolt 3. But then comes USB4.

USB4

While true Thunderbolt 3 SSDs are quite unusual, an increasing number of enclosures and complete drives now boast USB4 and that they’re “compatible with Thunderbolt 3 and 4”. There are two tell-tales to look for: these products only claim compatibility with Thunderbolt, and when you look at their USB-C port and case, there’s no sign of the official lightning bolt logo to indicate that they are genuine Thunderbolt products.

The confusion arises because, as far as data peripherals are concerned, USB4 is based on Thunderbolt 3, and offers a similar 40 Gbit/s. While USB4 drives are normally backward-compatible with USB 3.2 and 2.0 (which don’t help us at all), they can also be compatible with Thunderbolt 3, as an option. So a USB4 drive could be compatible with Thunderbolt 3, but won’t usually be marked as such.

The bad news for Mac users is that no Intel Mac supports USB4, although most recent models have full-blown Thunderbolt 3. The only Mac models with USB4 support are Apple silicon, which from the earliest in November 2020 have USB4 and Thunderbolt 3 support. You’ll see some otherwise accurate articles claiming that some Apple silicon Macs don’t support USB4, but as far as I can tell, that’s incorrect and they all do.

If we widen our search to include USB4, we could then end up with a drive that performs in the same class as Thunderbolt 3, until it’s connected to an Intel Mac, when it falls back to its USB 3.x support, and delivers meagre 1 GB/s transfer speeds, a third of those we see with an Apple silicon Mac.

Products

OWC proclaims that they’re “widely considered the Thunderbolt experts” and they’re probably the only vendor that makes clear these distinctions in their products. Those currently include:

• OWC Envoy Pro SX, with full Thunderbolt 3 support, but no USB 3.x fallback. This uses Intel’s Alpine Ridge controller (JHL6340) from 2016, and is marked with the lightning bolt. I can vouch for its performance.
• OWC Envoy Pro FX, with full Thunderbolt 3 from an Intel Titan Ridge controller from 2018, and USB 3.2 Gen 2 fallback at 10 Gbit/s from a Realtek RTL9210 controller. This too delivers Thunderbolt 3 performance with both Intel and Apple silicon models, and bears the lightning bolt.
• OWC Express 1M2, described as a “USB4 NVMe SSD”, available as an empty enclosure or with an SSD installed. When connected to an Apple silicon Mac, this delivers 40 Gbit/s to its Thunderbolt 4 port, but when connected to a USB-C port on an Intel Mac, OWC states that it falls back to USB 3.2 Gen 2 to deliver only 1 GB/s.

Of the enclosures currently available:

• Satechi‘s M.2 NVMe Enclosure is claimed to support USB4 but not Thunderbolt 3, and falls back to USB 3.2 Gen 2 when full-speed USB4 isn’t available.
• Acasis 40Gbps Tool-free M.2 NVMe SSD Enclosure appears to be another USB4 model, although it’s claimed to be “compatible with Thunderbolt 3/4”, with a fallback to USB 3.2 Gen 2. The vendor doesn’t provide information on its performance with Intel Macs.
• Orico‘s multiple models of SSD enclosures have in the past included one that was claimed to be Thunderbolt 3 and was marked as such. Their current range appears to be based entirely on USB4, and there’s no clear information about transfer speeds with Intel Macs.

Recommendations

• If an external SSD will only be used with Apple silicon Macs, then USB4 should deliver similar performance to Thunderbolt 3 or 4, and is more readily available, particularly as empty enclosures.
• For good performance and full support of SMART health indicators, SSDs that are also to be used with Intel Macs need to have official Thunderbolt support, with the lightning bolt logo. There don’t appear to be any suitable empty enclosures available, though.
• Thunderbolt 3 and 4 external storage is unlikely to become more available, and may well be replaced by USB4, which may or may not be compatible.
• If possible, buy external SSDs and enclosures on a full-refund basis, so you can return them if they fail to meet expectations.