Ejecting a backup device

Does it tell you it cannot be ejected, or do you just need to know how to eject a drive?

APFS backups have to do some reconciling after the backup completes, so that may prevent you from ejecting immediately. It should allow you to eject it fairly soon after, though.

Along with instructions from P. Philips, you can click the eject button in the Sidebar next to the drive name.

If you have Time Machine Backup Setup to >> Manually Backup << Just click the icon of the Backup Drive appearing on the Desktop and continue to hold it. Then Drag the icon to the Trash ( Bin )

No it does not delete the drive, it just properly dismounts the drive

I suggest you pause until the drive stops writing any reming data to the drive and then Unplug the drive

If the Time Machine Backup is Set to Automatic Backup >> on a schedule << - suggest you leave it attached all the time.

Otherwise, with the drive not attached, TM Backup will continue to make Snaps Shots on the Internal Drive which over time may fill up the drive leading to another issues ( Not Enough Space to make the Backup )

Back up your Mac with Time Machine - Apple Support (CA)

If you don't leave your backup disk attached at all time, you lose the ability of TM to create backups on a schedule. That means that data you create after your disconnect the drive, but before your reattach the drive is not backed up potentially being lost forever.