How can I fsck a partition when the device reads as busy (but has been confirmed otherwise)?

You can remove the partition from fstab and reboot the machine

There are things (usually in the kernel, like the NFS threads, swap files, bind mounts, etc.) that can keep a filesystem busy that won't show up in fuser.

If you try to fsck a filesystem that is mounted, it will get corrupted. You should find a live CD that doesn't automatically mount your filesystems, like Knoppix or Fedora.

Try booting another liveCD - I recommend SystemRescueCD that is designed specifically for solving problems with linux installations. It is also smaller than typical OS liveCDs. And to be on the safe side, run fsck from the console (no X).

If the standard fsck -p does not work, you can also try e2fsck -y -f.