It’s not that long ago that, every few days, I’d have to restart my Mac in single-user mode (SUM). It has been some years since I last did that, and – on this iMac Pro at least – I won’t be able to repeat it. Macs with T2 chips don’t do SUM any more, at least not in the same way.

Entering SUM is a matter of holding Command-S during startup. Instead of macOS booting up with its plethora of services and the GUI, the Mac stops far short and leaves you with just a command prompt. The most common reason for doing this was to run fsck to check and repair the boot disk, something which many users did after every hard crash. And those were not infrequent in early releases of Mac OS X, long before Apple invented Recovery Mode.

Even with Recovery Mode available, there have still been some reasons for wanting to enter SUM. One of them has been to run command tools to check memory for faults, as in SUM the system takes as little memory as possible, allowing you to run checks on all the rest. Sometimes fsck run from SUM could fix problems which Disk Utility in Recovery Mode couldn’t. But for many users, starting in Recovery supplanted any need for SUM.

One important difference between SUM and Recovery Mode are the tools available to you: unless you mount your normal startup volume in Recovery Mode, the only command tools you can access are those supplied as standard in the Recovery Volume, and that’s a deliberately very small sub-set of what you normally have available.

SUM started to get more tricky when Macs shipped with Fusion Drives. When formatted in HFS+, those use CoreStorage to manage the constituent SSD and hard disk, and that made fscking in SUM a much bigger gamble. If you weren’t very careful, and a little lucky, you could end up having to reformat the Fusion Drive from scratch. I think that’s probably when I stopped using SUM and fsck.

More recently, starting up with Command-R-S held has entered ‘SUM in Recovery’, which takes you straight into the command line, but it’s still fundamentally limited to what’s available in Recovery Mode unless you start mounting additional volumes. Some users have found that helpful, for example for changing SIP settings.

The biggest problem comes with Macs equipped with a T2 chip, and its Secure Boot. If it were available, a traditional SUM would bypass Secure Boot, so it isn’t allowed if your Mac has a T2 chip: Command-R will take you to Recovery, and that’s as close as your Mac can get, unless you disable Secure Boot by setting the Startup Security Utility to No Security. And you have to do that in Recovery anyway.

Apple details this here, and recommends that, if your Mac has a T2, you:

1. Start up in Recovery Mode, using Command-R.
2. Open Disk Utility, select your normal boot volume, and mount it using the Mount command in the File menu, if necessary.
3. Close Disk Utility and switch to Terminal.
4. Run your desired commands in Terminal, using tools on your mounted boot volume as required.

Although Apple states that this is true for “recommended settings on current Mac computers”, I see no reason that a current iMac (provided it doesn’t have a Fusion Drive) shouldn’t still support traditional SUM. But there wouldn’t appear to be many situations in which that is preferable over regular Recovery Mode.