Your server has a ZFS storage pool that is configured as follows: The server has...

You can convert a non-redundant storage pool into a redundant storage pool by using the zpool attach command.

Note: 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 configuration, 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.