Sorry to leap in again, but the umount is really needed. If you're checking a filesystem which might actually need repairing, it's critical that you either (a) umount it first, or (b) remount it read-only first, and then afterwards reboot without remounting it read/write. Anything else risks writeout of stale page-cache data undoing fsck's good work and/or causing complete filesystem scroggage.

On an Empeg (but not a real Unix box) you can get away without the reboot after checking the root filesystem only, but only because it hasn't actually changed, and the fsck just silences the warning by updating the last-check timestamp from 1999 or whenever it was we last rebuilt the filesystem images.

Peter