Sometimes you have to reboot the machine. Simple as that. Here are the right ways to do it. You need to be root for all but the last method...
If you ever need to use the L1-A key combination (also reffered to as Stop-A) to halt a machine, type sync when the ok prompt appears to save disk information.
This is important because it will help prevent disk errors which commonly occur when the machine is not brought down gracefully.
# sync
# reboot
You only need to do the shutdown step if you are intending on warning users. If you just want to be quick, just do a halt after the sync.
# sync
# shutdown -h now "---- Replacing a disk -----" (or)
# shutdown -y -g0 "---- Replacing a disk -----" (depending on version)
# halt
After a shutdown, you can probably just type boot at the bootprom level to bring the system back up again.
To boot into single user mode, boot -s.
To boot off of a specific disk, boot disk#, where # is the scsi id of the hard disk.
If you have hardware problems, you may need to shut off the computer and the associated external hardware (e.g. optical drives).
If you add new hardware, you should probably do a boot -r to reconfigure the system for the new hardware device drivers.