Mac storage basics · Time Machine basics
Does Time Machine back up my other external drives?
Short answer: Not by default — external drives are on Time Machine's exclusion list. Remove a drive from the list in Time Machine's Options to include it; it must be connected during backups.
Not by default — Time Machine adds external drives to its exclusion list. To include one, open Time Machine's Options… in System Settings and remove the drive from the exclusion list. The drive must be connected during backups for its changes to be saved.
Make sure your backup drive is big enough for the extra data.