Never trust a Time Machine made by a computer company

A fun discovery: if you have two Macs, one running Catalina and the other running Big Sur, you may struggle to back up the Big Sur machine to a network share on the Catalina machine because Catalina formats Time Machine volumes as HFS+ and Big Sur expects Time Machine network shares to be APFS. Catalina can write HFS+ Time Machine backups to an APFS volume, but not vice versa. The only reason I now know this is because I’ve read an excellent discussion in the Ars Technica review of Big Sur, not because Apple actually told me this might be a problem.

If you attempt to backup an APFS filesystem to an HFS+ drive on a network share, the Big Sur APFS machine will fail to set up a link to the Time Machine share. The error message it’ll give you isn’t “You are trying to back up an APFS filesystem to a non-APFS Time Machine filesystem, convert the HFS drive to APFS”, it’ll just complain that you don’t have the appropriate permissions to access the backup drive.

You can’t simply fix this by reformatting the HFS+ backup volume to APFS, because when Time Machine initiates on the Catalina host machine, it’ll reformat it back to HFS+, even though Catalina can backup to an APFS drive. The only way I got it to work was to format it as HFS+, wait for Time Machine to start backing up, cancel the initial backup, convert the drive to APFS, then Time Machine will warn you that the disk has changed,1 and you can restart the Catalina backup and setup the Big Sur backup.

Why not just upgrade both machines to Big Sur?

Another fun discovery: if you have a Mac that has a Fusion Drive,2 you can’t upgrade it to Big Sur.3 It’d be useful if Apple had told me this before I tried to upgrade the Fusion Drive machine to Big Sur. Instead, it told me about all the magical, wonderful shiny things that lay in store for me. Like spending my day off wiping hard drives, reinstalling Yosemite, then upgrading it to Catalina, then jiggling the relevant checkboxes in System Preferences until Time Machine finally whirred into action.

It just works. Until it doesn’t.

  1. “In what way?” you ask. You expect operating systems to give you information? Quaint. [return]
  2. For those who have forgotten: Fusion Drives are a small SSD and a big HDD that have been bound together into one volume. The OS uses Clever Magic® to redistribute files across the two drives. They were a good idea back when SSDs cost prohibitive amounts of money. [return]
  3. Some people on Reddit etc. have using a variety of nasty hacks. [return]