Question: Time Machine and external Drive warning
As information. If you want to test HS on a remote (external) drive while keeping your current OS on your internal drive be sure to disconnect TM before doing this. Here is what can happen: You start up your Mac on the internal drive, then change the start up disc containing HS & reboot. Then later you change your startup disc back to the internal and boot up. At this time TM will do a FULL backup (not incremental) filling up the TM drive. Not sure why this happens. This happened to me and was also reported by MacLife magazine April 2018.