Which external disk?

Over the last year, I have been testing many different types and models of external storage, ranging from portable USB 3.x hard disks to high-end Thunderbolt SSDs like the OWC Envoy Pro FX, costing from $50 to over $200 per terabyte. Individual reviews and group tests have appeared (and continue to appear) in MacFormat and Mac|Life magazines. This article gives an overall summary to guide your decisions.


I now conduct all tests using a Thunderbolt 4 port on the back of my Mac Studio M1 Max, running late releases of Monterey and, most recently, Ventura. Previous problems getting full performance from USB 3.1 Gen 2 devices appear to have vanished now, and I have seen no further performance anomalies during testing.

My standard test sequence is to connect the disk using its supplied cable to the port, erase the whole disk and format it with a single container and APFS (unencrypted) volume. I then run my own testing app Stibium, writing 10 repeats of Write Series, and one of Read Folder, without a restart between them. After that I run AmorphousDiskMark, with Test Count set to 5, and Test Size set to 1 GiB.

Once those are complete, I format just the APFS volume and install (currently) Ventura 13.0 using the full installer app. When that has booted successfully, I set the Startup Disk to that external disk and time how long it takes to reboot, from the startup chime to the appearance of the login window.

I make no attempt to measure SLC cache, or assess thermal throttling. Stibium’s write test writes over 50 GB to the disk at full speed, so I check that no drop in performance has occurred during that test, which implies that neither SLC cache exhaustion nor thermal throttling have occurred when writing such a substantial quantity of data.


If you want to see individual results for different brands and models, then you should refer to my published reviews. Here I’ll give an overall summary.

By far the fastest disks that I have tested are internal SSDs in Apple silicon Macs, with read and write speeds typically around 7 GB/s with full encryption. Nothing you can connect to your Mac comes close to that. Closest are high-end Thunderbolt 3/4 NVMe SSDs from the likes of OWC, which reliably reach transfer speeds close to the Thunderbolt 3 maximum of 3 GB/s. There’s then a strange group of Thunderbolt 3 SSDs that should be delivering better performance, but because of their electronics peak with transfer rates around 1.5 GB/s. With improvements in Intel chips and the changing market, those should gradually fade away.

When you drop from Thunderbolt to USB 3.x, the fastest rates you’ll see are around 1 GB/s, which is typical of better USB 3.1 Gen 2 models on an Apple silicon Mac. As far as I’m aware, no Mac in production supports USB 3.2 Gen 2×2 yet: that means any extra you might pay for an SSD meeting that specification won’t bring any significantly better performance until you’re using a Mac with that support.

More typical of USB 3.1 Gen 2 SSDs are transfer rates of around 0.8 GB/s, still well above those of SATA/USB 3.x, which generally struggle to exceed 0.5 GB/s, although that’s far superior to better hard disks at around 0.1 GB/s.

These become more meaningful when expressed in terms of speed per cost per terabyte, as shown in the table below.


Listed there are two self-assembled, both using Samsung 980 Pro NVMe SSDs, one with an ORICO enclosure that attains full Thunderbolt 3 performance, the other using a superbly engineered Sabrent enclosure. The Samsung SATA drive is also self-assembled using a cheap plastic enclosure.

Performance when booting Ventura is less clear-cut, and can vary considerably between disks with similar transfer speeds, suggesting that some use their caches more effectively. When booting from its internal SSD, a Mac Studio Max with Ventura should take around 12 seconds from chime to login. Several external SSDs support external booting in 16 seconds, including full-speed Thunderbolt 3 models and some USB 3.1 Gen 2 SSDs. More typical of SSDs are times of 20-30 seconds, with SATA SSDs slower still at 40 seconds, and hard disks as long as 100 seconds.


SSDs are much preferred over hard disks for storing Time Machine backups to APFS. This is because of the inevitable fragmentation that steadily degrades hard disk performance over time. Although backup storage is one of the remaining situations in which APFS can be used on hard disks, if your backups are substantial, you should expect to replace hard disks after shorter intervals, possibly even less than two years, and few hard disks are likely to survive much more than three years. SSDs can be expected to last 2-3 times longer.

It’s also important to remember that write speeds to hard disks typically fall as they fill up, when data is written to more central sectors on the platter(s). Outer sectors may have write speeds over 100 MB/s, but those near the centre may only achieve 60-70 MB/s, which is noticeably slower. With APFS, this occurs at the time that fragmentation is also growing worse.

Because macOS still throttles writing backups, there seems little point in using faster and more expensive SSDs to store Time Machine backups. Basic and relatively inexpensive SATA SSDs should deliver similar backup times to NVMe storage connected by Thunderbolt, at a fraction of the cost. High capacity is usually more important, and here you should consider investing in a multi-bay SSD enclosure, such as the OWC Thunderbay series, which can connect four SATA SSDs to a single Thunderbolt port. The alternative is using a hub with individual drives connected to it.

For more general use, SSDs delivering transfer rates of 0.7 to 1.5 GB/s should be ideal, and can cost as little as $75 per terabyte for high quality USB 3.1 Gen 2 drives. With transfer rates higher than that, costs rise rapidly to $200 per terabyte, in SSDs that are best suited to those with greater performance demands. Sadly, at present there’s a big price gap between high-end USB 3.x and Thunderbolt 3/4 drives. I hope next year’s new releases will include lower-cost Thunderbolt models to start filling that gap.

Apart from performance, there are other good reasons for preferring Thunderbolt 3/4 over USB 3.1 Gen 2. One of the less obvious is support for SMART health indicators: macOS has no support for SMART over USB, and there’s no easy way to address that at present. SMART is, though, fully supported over Thunderbolt, making it easy to monitor disk health without requiring extensions.

Finally, you generally get what you pay for. All the branded SSDs that I have tested are excellent, well-designed and beautifully built. OWC Envoy Pros are my particular favourites; while they might appear expensive, I’m sure many of them will still be appreciated for their speed and quality of build ten years from now.


  • Apple silicon Macs, when running late releases of Monterey, and in Ventura, do now realise transfer performance expected of USB 3.1 Gen 2, but don’t achieve the higher performance of USB 3.2 Gen 2×2.
  • All storage tested, including hard disks, can be used as an external boot disk, although SSDs and hard disks with transfer speeds below 0.7 GB/s should be expected to boot significantly more slowly.
  • SSDs should be preferred over hard disks for storing Time Machine backups on APFS. Hard disk performance and working life are seriously impaired when APFS is used.
  • For general use, USB 3.1 Gen 2 SSDs are an ideal compromise between cost and performance, but don’t support SMART health indicators.
  • Thunderbolt 3/4 SSDs enjoy full SMART support and deliver excellent transfer rates for more demanding use, but remain expensive.
  • Best transfer rates to be expected are around 0.1 GB/s for hard disks, 0.5 GB/s for SATA SSDs, 1 GB/s for NVMe SSDs over USB 3.1 Gen 2, and 3 GB/s for full Thunderbolt 3/4.
  • Shop around. There are bargains available, with excellent products from reputable manufacturers being sold well below their usual price.
  • Watch for new models next year, when Thunderbolt SSDs should improve further, and hopefully fall in cost too.