I would say it isn't worth the risk and leave it there until you've worked on the malware.
That said, I'm working on a system which is infected with the Locky blackmail and the backup drive was plugged in but is unharmed. Sooner or later the disgusting vermin who code this stuff will learn how to include all drivers and All Users.
I believe backup drives should always be away from the machine, preferably in a fireproof box.