Snow Leopard Mac won’t boot – startup disk full

or: what to do when your son tells you 10 minutes before he needs to go to school that the old family iMac is broken and he can’t get his work off that needs handing in like right now.

Symptom: old Mac running MacOS Snow Leopard 10.6.8 will get to login screen, but when you log in you get a blank desktop, no icons, no menu, just a mouse pointer.

Cause: the startup disk is utterly full despite like nobody downloading anything on it like, ever, oh my god.

Solution: using target disk mode wasn’t an option as we didn’t have the right adaptors or leads – the idea is you boot it up in a special way (hold down letter T on keyboard as it boots) so it becomes a Firewire or Thunderbolt disk, you mount it on another Mac, delete some files, disconnect, reboot. So that left me with the command line Single User Mode.

Here’s what I did. Rebooted the Mac holding down cmd S. This takes you to a command line where – if you are very careful and know what you’re doing – you can manually delete some files or folders to free up some space. What the first two guides I found didn’t tell me, however, is that this mounts the disk as read only, so you need to type sudo /sbin/mount -uw / to mount the drive in a way that allows you to delete files.

You can see how much space is free using df -h

Remove files using the rm command – I’m not going to explain how to remove folders because it’s risky – have a look online, and be careful!

Proceed with extreme caution. Luckily, I knew there was an old Dropbox folder, not updated in years, on my login so I was able to delete one folder and free up 2GB of space, enough to restart the Mac (type reboot).

Note that Single User Mode only works on old Macs – MacOS High Sierra 10.13 or earlier – presumably because this is a massive security hole allowing anyone to read your files.