Last Week on My Mac: Why M1 Macs don’t have full support for USB-C

Sat next to my iMac Pro is its successor, a Mac Studio (Max) with its matching Studio Display. But I’ve hit a snag: that new Apple Silicon Mac, like all other M1 models, doesn’t have full support for storage connected via USB-C. Specifically, it can’t access S.M.A.R.T. health indicators on any USB-C storage without downgrading its security. That forces me to choose between Full Security without health indicators, or Reduced Security with those indicators accessible. Surely Apple doesn’t really intend that?

Like so many unintended consequences, the origins of this quandary go back a long way, to 1995, when Self-Monitoring, Analysis and Reporting Technology or S.M.A.R.T. attributes were incorporated into the AT Attachment (ATA) standard to support monitoring of hard disk health. When Apple started to move away from SCSI interfaces for hard disks in 1996, ATA replacements in PowerPC Macs came to support S.M.A.R.T.

In 1998, with the first iMac, Apple introduced USB, enabling users to connect relatively inexpensive hard disks externally. Although monitoring S.M.A.R.T. over USB was and is possible, Apple decided not to support it. Apple made a similar choice when it added USB-C ports in 2015, and has never supported S.M.A.R.T. over USB-C either.

If you want support for S.M.A.R.T. monitoring over any form of USB connection, the only route has been a third-party kernel extension, SAT SMART, which is today still based on Jarkko Sonninen’s open source code last updated five years ago.

In the meantime, Macs and macOS have moved on. Third-party kernel extensions have been deprecated for years, and become a serious problem in Apple Silicon Macs. Apple actively encourages us to run these new models in Full Security mode, which forbids the loading of any third-party kernel extensions. If you want to load the SAT SMART kernel extension on an M1 series Mac, you have to downgrade its protection to Reduced Security. Thus, in failing to support the monitoring of S.M.A.R.T. over USB-C, Apple is giving us a stark choice: Full Security without S.M.A.R.T., or Reduced Security with it.

The simple answer might be to admit that M1 Macs don’t provide full support for USB-C storage, and that we should use Thunderbolt instead. The current premium for a 2 TB SSD is around $/€/£ 100, whether you buy a complete drive or assemble one yourself using an enclosure. But in many cases, that surcharge is largely wasted.

The most compelling reason for connecting external storage to a Mac is for backups using Time Machine (or other backup software). As no M1 series Mac supports adding internal storage suitable for those backups, the choice is locally attached or networked. Of course, a few years ago you might have considered a Time Capsule, but Apple discontinued those in 2018. Third-party NAS systems suitable for Time Machine are also limited by Apple’s deprecation of AFP, and at least one major manufacturer appears to be dropping Mac support as a result. If you’d prefer to back up online, you can forget iCloud, which still only supports the backup of devices, not Macs.

Assuming that your solution to this debacle of deprecations and discontinued products is locally attached storage, the only bus that enjoys full support by Apple is Thunderbolt, available for expensive RAID arrays of hard disks, or expensive NVMe SSDs. The latter could be attractive on performance grounds until you remember that Time Machine backups are performed with I/O throttling enabled by macOS, so you’re not going to see anything near the performance your expensive external SSD should be capable of.

My OWC Thunderbolt 3 SSD enclosure, which does enjoy full S.M.A.R.T. support despite housing SATA units, costs in excess of $/€/£ 1,200 for a total of 8 TB, the same as an entry-level M1 iMac.

So, Apple, which do you advise us to do, run our shiny new M1 Macs in Full Security without monitoring the health of their Time Machine backup storage, or downgrade their security and install a third-party kernel extension so we can keep a watch on the drive storing our backups?

There is an even better alternative: complete support for USB-C by investing a little engineering time to add S.M.A.R.T. health monitoring. Please?