"...can I instead use mount and unmount ( with a suitable macro or other timed automatic method) to isolate the backup drive from the computer, and only mount it to do the actual backup or update?..."
Since your question deals with backup and restore routines, I'll limit my comments to that; although Ranchhand's comments should be "listened to" and taken into account:
I do not know if there exists presently a ransomware that can re-mount, re-enable, an un-mounted or disabled external hard-drive or a 2nd internal hard-drive, respectively. I mention a 2nd internal HD because I do something very similar to what you are asking about -- I disable my 2nd internal HD after making my routine backups. If during our research, whoever first discovers, reads about, a ransomware that can re-mount or re-enable, let us inform the other.
Edited by RolandJS, 07 July 2017 - 06:04 AM.