Time Machine: one external drive does not show up in time machine's "exclusion" panel
Time machine panel where you choose disks to include/exclude for backup does not show one of my external disks. This disk has been previously backed up with Time Machine and I can see the backups of it when I enter Time Machine, but it no longer shows up in the panel where you exclude disks for backup, so I assume it will no longer back up that disk. This external shows up on the desktop, in Finder, and everywhere else, just not in the Time Machine Exclude panel. If I click the + button at the bottom of that panel, the external shows up in the icons for all disks, but not in the Exclude panel. The drive is connected by firewire, same ports as usual.
Mac Mini late 2012, 16 g ram, fusion drive, High Sierra
Questions:
- Is there any way to make Time Machine offer this disk for inclusion/exclusion?
- I am afraid to let Time Machine resume backups till I figure this out. Will it "lose" the contents of the disappeared drive? If so, how soon?
- Should I let Time Machine run and see if that drive changes in the backup, or is there some danger to that?
- Is there a good detailed learning resource about Time Machine?
Mac mini, macOS 10.13