The external drive I want to backup is grey-ed out and un-selectable.
Is it not possible to backup this drive? Why?
It is a SanDisk Extreme Portable 1TB Model: SDSSDE61-1T00.
Does it have to be reformatted or something?
MacBook Pro 15″, macOS 12.6
Sometimes you do need to reformat the external drive. Try this link if you need help.
Erase and reformat a storage device in Disk Utility on Mac - Apple Support
Your advice was sound but the apple documentation seems to be out of date at least for Catalina (12.x) OS. So I winged it. I Erased the drive (highest in the hierarchy for that drive, after View > Show all Devices) then Partitioned it. After that I was able to remove it from the Time Machine Options>Excluded list.
Time Machine has Excluded external drive I want to backup; grey-ed out and un-selectable.
