problems setting up disk to use with time machine
I have a set up with 3 Macs. I have a sata docking station attached to one of them which I use for time machine. This allows me to easily take rotate physical disk for off site storage. I have used this system for years. I back up multiple machines to one disk.
I recently add a new disk and followed instructions (that I now can't find) to format the disk as APFS (encrypted) shared it with the other machine on the network and did a backup. So far so good.
I now tried to back up the machine that had the disk directly attached. When I tried to select the disk in Timemachine prefs I got the familiar prompt "Use new disk or both", selected both and was then told that I had to erase the disk!
one can see the here:
1/ the formatting information from disk utility
2/ the dialogue from Time Machine prefs
In fact I have been around this process several times : (. I have tried backing up the local machine first and then it won't let me backup the remote one. I have tried formatting the disk with Journaled... like my older disk, but nothing works for both machines.
side note: As mentioned this all used to work fine. About a year/ 18 months ago it the disk sharing broke after a major OS upgrade (I can't remember which). I eventually concluded that the issue was that the UIDs (unix numeric ids) of account were different. Fixing this is not straight forward and I put it off and eventually did it recently. I suspect the change was a consequence of Apple attempt to make it more difficult for malware to modify files.
Suggestions solicited!
iPad mini 4, iPadOS 14