Last Week on My Mac: Storage stories

Selecting and buying storage has never been straightforward. In the days long before SSDs, there were plenty of disasters: there were IBM’s ‘Deathstar’ hard disks that crashed and burned with regularity until Hitachi transformed them, and Zip drives that suffered the ‘click of death’; then came tiny Flash drives, that often wrote small files at less than 1 MB/s, and SSDs that corrupted their contents when trimmed.

There has always been a big difference between what’s available, and what’s advisable. Current Macs provide basic support for a wide range of different SSDs suitable for quite different purposes, from affordable models with basic SATA interfaces using USB 3.x, up to high-performance NVMe over Thunderbolt 3 if you’ve got the money. To expect them all to support the same features is perhaps more than a bit optimistic.


Early problems with Apple silicon Macs getting expected performance from USB 3.x SSDs seem to have settled now, and most if not all Thunderbolt 3 drives can use all four lanes of PCIe, giving them a clear edge with transfer speeds of 2-3 GB/s against 1 GB/s of USB 3.x. This could of course change with the arrival of USB 4 SSDs, but as those are necessarily compatible with Thunderbolt 3, it seems unlikely that they’re going to be any cheaper.

Looking at SMART health indicators and support for TRIM becomes far murkier, though.

Health monitoring

Monitoring the health of external storage on the Mac has been a problem for years. The SMART (or S.M.A.R.T. if you’re pernickety) standard makes even USB look tight and prescriptive, and Apple’s support has been disappointingly patchy: FireWire and Thunderbolt generally, but never for any version of USB. Yet third-parties who try to provide better support can only do so for SATA and not NVMe, because of the kernel extension we still have to rely on. The end result is that SMART health indicators on most external storage are inaccessible to Macs.

Those who put little trust in SMART may be unconcerned, but there are plenty of users who would rather have something than nothing. What is contradictory is that you can’t get these from an NVMe SSD connected by USB 3.x, but attach the same SSD by Thunderbolt and macOS is happy to get them for you. So if you do want to be able to see any health indicators for an SSD, it really has to be connected by Thunderbolt.

TRIM support

It gets still worse when you consider TRIM, for which there has been a dearth of information for the last seven or eight years, a period in which SSDs have undergone great change. Again, the only SSDs that macOS considers consistently support TRIM have an NVMe interface and are connected by Thunderbolt.

This is strange, as most SSD manufacturers like Samsung claim that all their current products support TRIM, although they don’t seem to support Macs at all. Apple seems to enable TRIM on its own internal SSDs, but leaves it to SSD suppliers to advise users whether it should be enabled for their external drives. Yet Samsung and others tell Mac users to contact Apple for advice about configuring their storage products.

Maybe TRIM support is a complete red herring, and doesn’t matter any more. If that’s true, it doesn’t explain why APFS performs a trim on some SSDs during mounting, including Apple’s own internal storage, but not on those connected via USB 3.x or with SATA interfaces, many of which it declares don’t support TRIM.

To give practical examples, I have four Samsung SATA 860 EVO SSDs in an OWC ThunderBay 4 enclosure; macOS supports their SMART health indicators well, but none of them has TRIM support, despite Samsung’s assurance that they do. Buy any of the justly popular USB 3.2 SSDs with a PCIe interface to deliver up to 1 GB/s performance, like Samsung’s T7, and it has neither SMART nor TRIM support when connected to a Mac.

You get what you pay for

If we draw up specifications for what we want in an external SSD to last for the next few years, it’s surely going to need consistent performance, access to SMART health indicators, and TRIM support. Those require it to have an NVMe interface and connect using Thunderbolt 3. Priced on a self-assembly 2 TB model, that starts at around $/€/£ 230, or slightly more than $/€/£ 100 per TB. For little more than half that price, at around $/€/£ 60 per TB, I can buy a 2 TB NVMe over USB 3.2 SSD offering almost 1 GB/s, but with neither SMART nor TRIM.

Selecting and buying storage still isn’t straightforward.