1. what kind of fs do you believe is on 'sdu'?
2. what does /etc/fstab look like?
3. you don't mount disks, you mount partitions eg sdu1, not 'sdu'.
4. try these cmds
# need to be root here & that's a lowercase 'L' btw
BTW, RHEL5.x is now up to 5.10; you might want to consider updating for security & bug fixes.