I think the top level directory has to exist (or the target of the
symlink if you use one) for that to happen. I'm not sure if that's
the case for devices that udev mounts by label names or not. That
is, if the mount point directory goes away with the device.
In any case it might be worth setting a a cron job to stop backuppc
and send a notification if the disk is not present when it is