Time Machine Backup Questions

I recently used Time Machine to migrate my system from an old iMac to a new M4 Mac mini. The migration went well, but I have a couple of questions regarding Time Machine. After the migration, Time Machine seems to be making backups. I have an external HD dedicated to only Time Machine Backups. I also have another external HD containing Pictures, music etc that was being backed up when the Time Machine HD was attached to my iMac. Now that HD is connected to my Mac mini, and I'm not sure the other external HD is being backed up. When I go to System Settings>General>Time Machine>Options is see the following message:

Which I think is telling me it G-Drive USB-C is not being backed up. I haven't changed any of the Time Machine setting since migrating to the Mac mini, so apparently /Users/Shared/adi and Time Machine BU are not being backed up, which I think is correct. They shouldn't be. I also noticed on my Time Machine BU HD, in the Backups.backupdb folder there are now two subfolders one for my old iMac, and one for the Mac mini. Should I change anything in the "Exclude for Backups" window? Can I add the G-DRIVE USB-C HD so that too will be backed up using Time Machine?

Thanks for reading this long post. I appreciate any advice to insure Time Machine is working properly.

Ed