Time Machine: Include files and folders from another drive
If you set up Time Machine the first time, it automatically excludes any external drives including the Time Machine drive itself.

If you want to have certain drives included, just remove them from the exclusion list inside System Preferences &gt; Time Machine &gt; Options and they are backed up the next time Time Machine runs.

For more information, check out Apple's official support document: Mac Basics: Time Machine backs up your Mac