backing up external drive with time machine, cannot select external drive
I had to move my music app library to external drive connected to my Mac Mini because I have the small HD in the mini (251 GB). Despite some bumps along the way, it's done. However, I can't back up that library with Time Machine. The drive appears in the window in Time Machines options, but is grayed out, just like the external drive for the backups.
Years ago, I had backed up a firewire external drive via Time Machine on my old iMac (iTunes library). I've seen various videos and articles describing the process of including an external in Time Machine backups. Nothing I've experienced or seen mentions the problem I'm having, the inability to select the drive at all in Time Machine System preferences. Any ideas would be greatly appreciated. I have a lot of old CDs that I imported into my old iTunes library. If the external drive would fail, I'd lose that. And I don't want to buy a third hard drive to just back up the music library. Running LaCie drives for both music and Time Machine
thanks in advance
Mac mini, macOS 12.6