[ILUG] vanishing disk space

[keith hyland]

i recently moved a couple of pata EXT3 formatted disks from a CentOS machine
to a newly built openSUSE 11.3 box. - the first time i mounted them the
sizes looked about right, but after running chown on /mnt/srv to adjust the
ownerships, /mnt/srv has lost 20GB and run out of space. the chown command
completed correctly.

/dev/sdc1            480590764 459308361         0 100% /mnt/srv
/dev/sdb1            480590764 417037192  39134372  92% /mnt/backup

/var/log/messages is empty - syslog wasn't installed at the time.

fstab reads:
/dev/sdc1            /mnt/srv             ext3       defaults              1
2

mtab reads:
/dev/sdc1 /mnt/srv ext3 rw 0 0
/dev/sdb1 /mnt/backup ext3 rw 0 0

i haven't written to /mnt/backup - as the name suggests, its a backup of
/mnt/srv *but* its not up to date, so destroying /mnt/srv is the worst
option.
browsing the file structure with mc shows the correct space, free space of
20GB (4%) and free nodes 60917884 (29%) of 61048832
 - at a glance those node numbers don't seem to make sense though...

short of buying another 500GB disk and copying everything over, what options
do i have to restore the drive ?