including an external drive's contents in Time Machine backup
I read that it is possible to include an external drive along with my Mac's hard drive in the Time Machine Backup. When I go to the Options in Settings, however, the external drive is greyed out and cannot be selected, never mind removed from the exclusion list. The external drive in question is a SanDisk 1TB SSD. Is there any way to remedy this? Screen shot included: Backup Plus is the Time Machine drive, ExternSSD is the drive I want to include but can't.
MacBook Air 13″, macOS 12.1