Thanks to all for the suggestions.
Mount options are unchanged. I can post my fstab if it helps?
Ran a boot time fsck but the problem still exists. Is there a log file I should be looking at? Google reckons /var/log/messages, but that file doesn't exist on my system. I have /var/log/fsck/checkfs and /var/log/fsck/checkroot, but they haven't been modified since October '11. I'm running Mint 12.
ls */ gives:
lsof | grep FolderFour gives nothing.
Any more ideas?