I just replaced a drive in my pool the other day. I'm expanding my pool by re-silvering in larger capacity drives.
AFAIK when you replace a drive that currently is "online" and healthy, you must choose the "force" option
when doing it this way. Once the re-silver is complete, FreeNAS will off-line the replaced drive. At that point all you
need do is shutdown the machine and pull that drive, then reboot. PLEASE NOTE: Your drive labels will change once again!