Log off and log in as root, move all the data from /home to /mnt/disk, unmount /mnt/disk, change the /etc/fstab, mount /home:
# mv /home/* /mnt/disk
# umount /mnt/disk
Assuming /mnt/disk is on /dev/hda3, change /etc/fstab from:
to
then
# mount /home
Log off and log on as a normal user where your home directory (and all other users home directory) will be on the /home partition rather than the /home directory on the root partition.