Whoever thought it would be a good idea to make Windows backup dependent on a small MediaId.bin file residing on the backup target, outside of the actual backup directory, oughta get fired.
So, let's say you're using a samba drive as your backup destination, where there already exists a bunch of directories. Windows will create its own WindowsImageBackup directory in there, and that's fine.
Then, a few month later, you're cleaning up the root of your backup directory, and seeing this small MediaID.bin, which doesn't seem to serve any purpose (if it's that small, it can't be that important - plus all it really contains is a 16 bytes payload).
Bad move: now NONE of your backup archives will work.
If you need a unique ID to identify that a backup file is legitimate, how about you add it to the backup file itself instead of creating an extra MediaID.bin, outside of the backup directory, that screams "delete me"?