Occasionally I will manually offline a disk in my ZFS pool for one reason or another. Annoyingly I will sometimes get this error when I try to online that same disk back into the pool:
cannot online /dev/sda: cannot relabel '/dev/sda': unable to read disk capacity
The fix, thankfully, is fairly simple. Simply run the following command (make double sure you’re doing it on the correct device!)
sudo wipefs -a <DEVICE>
After I ran that command ZFS automatically picked the disk back and resilvered it into the pool.
Thanks to this superuser.com discussion for the advice!