Time Machine disk with Big Sur and the older macOS

I see that there are many posts about Big Sur and Time Machine. However, I have a case with a combination of Big Sur and other machines. This is pasted from my blog note [1].

Conclusion: using one external disk for Time Machine backups from several machines with several macOS, including Big Sur is possible with partitioning. But how about the AirPort Express Time Capsule unit?

Up to macOS 11 Big Sur  (now it’s “Mac OS X” (again): versions), ie. from some time back to 10.15 Catalina then Time Machine disks needed to be formatted as HFS+ (HFS Plus or HFS+ or Mac OS Extended or HFS Extended) – but NOT AS case-sensitive.

With Big Sur this changed. Now the “default” (stronger: it looks like it’s the only format allowed) format is APFS Apple File System. This is how Big Sur told me (it was in Norwegian, so I’ll try to translate):

Time Machine Could not complete backup to “myNetworkAirPortExpress.local”. 

Time Machine could not backup up the disk “%$” because there could not be made a snapshot of the disk.

“%$” for my AirPort Express station indicated something really bad! Some coding error if you ask me. I repeated this over and over. No help. It didn’t repair itself. Of course I had no clue about the change of disk format required. Yet.

But it started to dawn on me when I got the below error message, when I removed, from the Time Machine panel, one of the external disks I had used as an additional Time Machine backup disk. This one had been plugged into a myNetworkTimeCapsule (just making up names so that you may understand what I have) USB slot. But now I moved it to the Mac Mini‘s USB slot instead. That was when I tried to add it again, in the Time Machine panel. The Mac Mini machine run macOS Big Sur:

Do you want to delete “myExernalDisk” so that it may be used with Time Machine?

Deleting destroys all information on “myExernalDisk” and cannot be undone.

The volume has to be deleted because it does not differentiate between small and capital letters (is not case-sensitive) while some of the volumes that is backed up, do so.

But I used that volume for Time Machine backups from two additional machines: One with Mojave (10.14) and one with High Sierra (10.13). Solution: use Disk Utility and partition the disk! One partition as APFS and one as HFS+. No, and then yes!

The first partition I did, was with the Big Sur (11.0.1) version of Disk Utility. I needed about 500 GB for APFS and 1,5 TB for HFS+. That’s what I told it. But this turned out to become 1 TB for APFS and 2 TB for HFS+ = SUM 3 TB on a 2 TB disk! Unbelievable!

The APFS Time Machine backup went ok. But when I tried it on one of the other machines I got the fantastic message that it needed some 500 GB but had only some 800 GB! Some flaw with calculation of the 1 TB that wasn’t there at all.

I then started Disk Utility on the Catalina 10.15.7 machine. I had in the meantime updated it from Mojave 10.14. I partitioned it at about 500 GB and 1500 GB again. All looked fine until I discovered that when I pressed the button to do it, I could see the values go to the disk being divided into two equally sized partitions. About 1 TB each. At least the sum was right!

Now 2 = 1 +1 TB partitioned disk takes Time Machine backups from Mojave, Catalina and Big Sur! Happy!

However, my AirPort Express and AirPort Time Capsules are lost for Time Machine backups from Big Sur. Or may it be refomatted?

[1] https://www.teigfam.net/oyvind/home/technology/059-some-osx-notes/#time_machine_disk_with_big_sur_and_the_older_macos (disclaimer: no ads, no money, no gifts - only fun and expenses with my blog)