Attaching and Detaching Devices in a ZFS Storage Pool
In addition to the zpool add command, you can use the zpool attach command to add a new device to an existing mirrored or non-mirrored device. For example:
# zpool attach zeepool c1t1d0 c2t1d0
If the existing device is part of a two-way mirror, attaching the new device, creates a three-way mirror, and so on. In either case, the new device begins to resilver immediately.
In is example, zeepool is an existing two-way mirror that is transformed to a three-way mirror by attaching c2t1d0, the new device, to the existing device, c1t1d0.
You can use the zpool detach command to detach a device from a pool. For example:
# zpool detach zeepool c2t1d0
However, this operation is refused if there are no other valid replicas of the data. For example:
# zpool detach newpool c1t2d0
cannot detach c1t2d0: only applicable to mirror and replacing vdevs