Expanding a zpool to the entire disk space

zpool online -e zpoolName /dev/disk/by-id/diskZpoolIsOn

Using the device names (like sda) would result in this command silently failing, which is why the disk id is used

Adding an extra disk as a mirror

zpool attach zpoolName /dev/disk/by-id/existingZpoolDisk /dev/disk/by-id/newZpoolDisk

Removing a disk from a zpool

zpool detach zpoolName /dev/disk/by-id/zpoolDiskToBeRemoved