> Back up an external drive
How should I back up the media libraries I store on my external hard disk?
Media libraries and document archives tend to change far less often than active parts of your Home folder, such as Documents, and normally don’t merit backing up every hour, as Time Machine would do by default. Unless you add many photos during the course of most days, they may not even need to be backed up daily, but just once a week.
While Time Machine in recent versions of macOS does offerbackups of that external disk, to a different folder on your backup drive.