Partimage is not designed to back up a system that is in use. If you want to backup the FC5 system, you should boot to a livecd like pclinux or boot into another Linux system and run partimage from there. You need to unmount any Linux systems before using partimage to save that system. Do that with the umount
If you want to make a backup of FC5 while it is running, I use cpio
I have another partition which I save my backups to.
My FC5 box is located on /dev/sdb2 and I want to save the backup image to the backup folder located on /dev/sdb1.
As root user, I make a folder called /mnt/backup.
Then, I mount that folders
mount /dev/sb1 /mnt/backup
Now, I save the FC5 system to a compressed image in the /mnt/backup folder.
(cd / ; find . -xdev | cpio -voc > /mnt/backup/linux.cpio.gz)
Note: To restore that image, you will need to boot up with a livecd or other Linux system.
While in that other Linux system, don't forget to unmount the Linux system that you want to backup. Also, I format that partiton before doing the restore.
mount /dev/sdb2 /mnt/mudd
mount /dev/sdb1 /mnt/backup
(cd /mnt/mudd ; cpio -vicdm < /mnt/backup/linux.cpio.gz)