Issue the command 'sysconfig -r' has the drive been failed? You can also find that out by reading the messages log (Command: rdfile /etc/messages).
If the drive has already been failed by the system it has been spun down & is ready for replacement. You can remove the drive from it's bay & insert the new drive. Hot swap the drive. You do not need to power down the shelf, power down the controller, nor reboot. Enter the command 'sysconfig -r'. Do you see the drive in your spare disk pool? If not you need to assign disk ownership. The system will automatically assign disk ownership if the disk autoassign option is enabled. Issue the command 'options disk.auto_assign' to verify that it is turned on. If not you will need to issue the command 'disk assign all' or disk assign disk.name' (ex. disk assign 0a.01.1). This will assign software based disk ownership to the controller you issue the command from. A reason you may want to manually assign the drives is if you add a new shelf & you want to divy up the drives evenly to each node. So in all actually there are really one or two steps.
If the disk.auto_assign option is on:
1) Hot Swap the drive.
If disk.auto_assign option is off:
1) Hot swap the drive.
2) Assign disk ownership with the 'disk assign' command.