Back up Photos library that exists on external hard drive
I currently store my Photos Library on an external hard drive and want to create a backup of that library. I currently use a different external hard drive for Time Machine backups and thought I was backing up the library onto that hard drive but I don't see it there. What is the best way to create a backup of the Photos external hard drive? Open to any and all suggestions for Photos storage and management of backups. Thanks!
MacBook Pro (Retina, 13-inch,Early 2015)