Steve Fosdick lists@pelvoux.nildram.co.uk writes:
mount /dev/hdb5 /usr
then update /etc/fstab to mount /usr from hdb5 in future.
...so you'll only discover that you edited /etc/fstab wrongly next time you reboot, rather than immediately.
If you edit fstab and then just do "mount", not only do you detect mistakes early, you have less typing to do as well.