While Mints can now tell you whether your expensive new SSD gets trimmed by your Mac when it’s mounted, you really want to know which features are supported by different SSDs before you pay for them. With the aid of a couple of Macs, one Intel T2 and the other Apple silicon, over a dozen assorted SSDs, and the occasional kernel panic, I can now present my conclusions, as of macOS Ventura 13.3.1.
As far as your Mac is concerned, each external SSD has three important properties:
- the SSD itself, e.g. Samsung 990 PRO,
- the bus interface of the SSD, either SATA or NVMe,
- the connection to the Mac, either USB or Thunderbolt.
All reasonably recent, better quality SSDs should support the TRIM command or an equivalent, garbage collection, and SMART health indicators. Manufacturers should provide this information, although it can sometimes be hard to find. What few reveal is how these are implemented, and the level of support for different SMART indicators.
If you’re unable to satisfy yourself that the SSD has adequate support for these, then don’t buy that brand or model.
This is important for SSDs used with Macs, as it determines whether macOS recognises them as having TRIM support, and whether it will obtain and monitor SMART health indicators.
SSDs with a SATA interface don’t enjoy TRIM support as standard, and only provide limited SMART support when they’re connected by Thunderbolt, and none via USB. Over Thunderbolt, their SMART support is limited, in that Disk Utility reports their verification,
diskutil gives no detailed information, but dedicated apps like DriveDx should be able to provide a useful range of health indicators. If you have to use SATA SSDs, then putting them into a Thunderbolt enclosure like an OWC ThunderBay 4 does at least provide some SMART access, although it still doesn’t enable TRIM support.
NVMe SSDs are better off, as they appear to enjoy TRIM support whether they’re connected by USB or Thunderbolt, although they only have SMART support when connected by Thunderbolt. As the SATSMART kernel extension only supports SATA and not NVMe, there’s currently no way to gain access to SMART indicators for NVMe SSDs connected by USB.
By default, no SSDs connected by USB have SMART support, and only NVMe SSDs can support TRIM. SMART support can be added for SATA SSDs using the SATSMART kernel extension, but on Apple silicon Macs that requires reduced security.
A Thunderbolt connection should provide NVMe SSDs with full support for both TRIM and SMART, but for SATA SSDs TRIM still isn’t supported, and SMART indicators are more limited.
The current version of System Information is confusing and incomplete when it comes to TRIM support:
- SATA SSDs connected via Thunderbolt have TRIM support listed in the SATA section.
- NVMe SSDs connected via Thunderbolt have TRIM support listed in the NVMExpress section.
- All SSDs connected via USB should have TRIM support listed in the USB section, but don’t.
Where System Information does report that TRIM is supported, this matches reports in the log by APFS of trimming SSDs when mounting them.
If you want an SSD to have full TRIM and SMART support, the only suitable combination is:
- an SSD with good TRIM and SMART support,
- an NVMe interface,
- a Thunderbolt connection.
You may also be able to attain that for a suitable SSD with a SATA interface over USB if you:
trimforceto enable TRIM to external SSDs,
- install the SATSMART kernel extension to enable SMART support.
Before considering enabling
trimforce, ensure that you read its man page.
One popular combination can’t offer SMART support under any circumstances, but should support TRIM without the use of
trimforce: an NVMe interface with a USB connection. These are typically USB 3.1 Gen 2 models claiming transfer speeds in excess of 600 MB/s.