Any external non-TM filesystems are added to the exclusion list by default when a disk is connected, as by default TM wouldn’t backup externals. You would normally remove this filesystem from the exclusion list to allow backups.
Additional filesystems on the internal disk do not appear in the exclusion list by default, and so would be included in the backup if no further customisation were performed.
Bear in mind though that a backup to the same physical device is not really suitable as a backup. If you lose the disk (through either physical loss or corruption), you lose both the backup and the originals. Instead if you keep originals and backup separate you have better odds when you (inevitably!) lose a device.