> System Administration Commands zpool(1M)
>
> [...]
> zpool attach [-f] pool device new_device
>
> Attaches new_device to an existing zpool device. The
> existing device cannot be part of a raidz configuration.
> If device is not currently part of a mirrored configura-
> tion, device automatically transforms into a two-way
> mirror of device and new_device. If device is part of a
> two-way mirror, attaching new_device creates a three-way
> mirror, and so on. In either case, new_device begins to
> resilver immediately.
>
> -f Forces use of new_device, even if its appears to
> be in use. Not all devices can be overridden in
> this manner.
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