Note: If the box contains critical data you should perform a backup before following these steps.
First of all I recommend to run a reboot and simply enforce another fsck to see if the issue persists:
shutdown -rF now
This will perform an fsck for all partitions, so it could take a while. If the issue persists you could boot the system from any bootable live CD to perform the checks. Another option would be to boot into single user mode and trigger the check. You need console access, then reboot the box.
When you see the "boot:" prompt type "tab".
You will now see a list of kernels, such as
Type in the name of the kernel followed by "single":
Log in as root. Now no processes are running and you should be able to unmount and check the partition.