["Followup-To:" header set to comp.unix.questions.]
What command are you using and what is the exact error message? A drive
is supposed to be unmounted when you run fsck. You can't check the
amount of free space on an unmounted drive. If /dev/u is not mounted,
"df /dev/u" will show the amount of free space on the filesystem where
/dev is located, probably / on an SCO system. (/dev is a separate
filesystem in current Linux distributions).