root can not change the permission of old oracle archive directory in solaris 8

I am using oracle 9i with sun solaris 8 on Sun E 250 server.earlier we are taking backup of oracle archive files on /orabackup directory.in which archive file are stored with ...arc.Z extension. now we have changed the archive backup directory.now when we are trying to delete earlier directory /orabackup or files in this directory with root login ..it says PERMISSION denied.
when we try to change the permission from Chmod or ownership by CHOWN with root login it says....warning...permission deined... the /orabackup directory has consumed atleast 30gb harddisk space..

wud u plz tell me how can i delete the complete directory /orabackup.

thanks...

your early response wud be higly appreciatiable

Post the output of these commands:
df -n /
df -n /orabackup
df -k /
df -k /orabackup
ls -ld /orabackup
grep orabackup /etc/vfstab

mv /orabackup /orabackup_old
chmod 0 /orabackup_old
rm -rf /orabackup_old

Could it be in an NFS mount where root is translated to nobody?
Answering Perderabo's questions would help deduce that.