Good news – there’s a handy tool in OS X called wait4path which can help when writing timed scripts to backup to removable media.
Bad news – it [at least in Tiger….] works slightly esoterically – if a path is already present, it will still wait for another mount kevent before exiting. It should therefore be used in a script like this:
if [ ! -d /Volumes/Backups ]; then
echo "waiting for backup volume..."
# do some backups
Note, however, that if you do this in a crontab job it could potentially wait for a very long time, so you should wrap all that with a /var/run style semaphore.