Thanks for this tip! I've been achieving this by pointing a symlink at a subdirectory that only exists when the external drive is mounted, and having my backups target that symlink. But this seems cleaner.
Me too. Not sure why using an immutable mountpoint is better than a possibly broken symlink, both result in a write failure. I like the symlink, because it's visually obvious when it's broken in the shell, and I can keep my real mounts under /mnt or /pool etc while also putting the target (symlink) in /home/user.
