Time Machine will not back up external drives on my Mac Studio
I have a Mac Studio with an internal drive, plus 3 external drives: 2 drives for photos etc, 1 exclusively for Time Machine backups. Time Machine backed up the contents of the internal drive plus the 2 external drives for photos, to the dedicated backup disk, up until May 2024. Since then it has not backed up the external drives.
When I open a finder window for the external drive (for example, the drive MV4), and then choose "Browse Time Machine Backups" from the menu bar, a stack of windows appears with a timeline at lower right, as I'd expect. The top window shows "Today (Now)" and the topmost directories in MV4, as expected. When I press the upward arrow just to the right of the stack, the underlying windows appear, all with "MV4 primary@snap-133665" at the top, and no directories or files as contents. The directory "2024-08-16-164529" on the backup disk, for example, lists a directory "MV4" but it contains nothing.
All disks are APFS. External drives are connected to USB ports on the Mac Studio.
The internal drive has 600 GB free. For some reason it's formatted as 5 volumes. Not sure that's relevant. It's a feature I'd like to get rid of.
Mac Studio, macOS 14.6