[dnanian]

I have no other suggestions for you, Izzy: naming the drives the same way should work fine (and does for me, when I've tested the same solution).

Perhaps you can try to see what files are open on the other drive. To do so, run:

sudo lsof | grep "/Volumes/The Original Drive Name 1"

in Terminal. Note that "1" is because your backup drive would be named the same.