hoakley Macs, Technology

Which external drives have Trim and SMART support?

When selecting an external drive, particularly an enclosure for an SSD, there are two features that you’ll want: Trim support, and access to its SMART health indicators. This article explains which drives support them, and how to check.

The two key facts that determine whether a Mac supports Trim and SMART are the connection type or bus, and the interface used by the storage. I’ll take Trim support first.

Trim

This is housekeeping performed on an SSD in response to a command sent by the Mac’s file system. It’s named after the TRIM command used with SATA storage devices, and on others may use a different command such as UNMAP. This enables the SSD to erase storage blocks that are no longer in use by the file system, so freeing them for reuse. Although most SSDs have additional systems for doing this, if an SSD accumulates too many unerased and unused storage blocks, its write performance can fall dramatically until it has been properly trimmed.

In normal circumstances, APFS and HFS+ will only Trim SSDs with an NVMe interface. Those with a SATA interface are normally shown in System Information as not having Trim support. macOS does have a command trimforce that some use to force external SSDs to be trimmed, but it’s not clear how well this works. At least for faster SSDs, you should aim for them all to have Trim support without relying on trimforce to enable it.

If you find the right section in System Information, you should be able to discover whether Trim is supported, and that appears a reliable indicator when you can locate it.

If you’re unsure, or want to confirm that any given SSD can be trimmed properly, it’s simple to perform a test using my free utility Mints. In essence, you unmount a volume on the SSD, then mount it again at a known time. Mints’ Disk Mount log extract for the first few seconds after mounting the volume again will then contain an entry reporting that trimming has been performed, which it should be when mounting any APFS or HFS+ volume. Full details are given here.

SMART

SMART health indicators provide detailed information about how heavily a drive has been used, and whether there are signs of its imminent failure. These are more controversial, and some consider them unreliable, although usage indicators, temperatures and other data from an SSD or hard disk are of significance even if they aren’t accurate predictors.

Without installing any third-party extensions, macOS can only access SMART indicators over FireWire, Thunderbolt and USB4 connections, and not over USB 3.x or earlier. There is a free kernel extension, the SAT SMART driver, that can open access to SMART indicators on SATA disks connected via USB 3.x and earlier, but those don’t work with NVMe disks, apparently.

There are several places you can check SMART support:

  • Disk Utility
  • System Information
  • A SMART utility such as DriveDx
  • Mints, in its Disk Check feature.

Unfortunately, these may not agree with one another, as System Information may claim that SMART status is verified on drives that don’t and can’t support it.

Hybrid drives

With the arrival of USB4, enclosure manufacturers are increasingly building hybrids that can perform well using two or more different protocols. In some cases, this is achieved using two chips, perhaps one supporting Thunderbolt 3, the other USB 3.x. I’ve just been testing OWC’s Express 1M2, which uses asmedia’s ASM2464PD chip to deliver what is described as “USB4/Thunderbolt” with USB 3.2 gen 2 as its fallback. That functions at a full 40 Gbps over USB4, and drops to 10 Gbps when only USB 3.x is available, and those two determine Trim and SMART support.

When connected to an Apple silicon Mac, with its USB4 and Thunderbolt 4 support, the 1M2 runs in USB4 40 Gbps mode, with full support for both Trim and SMART indicators. But with an Intel Mac, which lacks USB4 support, its fallback of USB 3.x still supports Trim, as it has an NVMe SSD, but not SMART, as its connection is USB rather than Thunderbolt.

With the 1M2 connected to an Intel Mac, System Information’s USB section claims that its SMART status is “Verified”.

textssd1

Disk Utility differs, though, stating that it’s “Not Supported”.

textssd2

And DriveDx and Mints agree.

textssd3

Thus, System Information shouldn’t be relied on when checking for SMART support.

As hybrid drives become increasingly popular, and are offered by a wider range of manufacturers, this is only going to become more complicated. Please feel free to scream now.

Summary

  • Trim is normally supported for NVMe SSDs, but not for SATA.
  • SMART health indicators are normally supported over FireWire, Thunderbolt and USB4 connections, but not USB 3.x or lower.
  • Support of hybrid drives varies according to the interface and connection in use at that time, and may well differ between Intel and Apple silicon Macs.
  • System Information can’t be relied on when reporting whether SMART is supported: it may be wrong.
  • USB4 makes this even more complicated.