For an active pool

If this error was encountered while running 'zpool import', please see the section below. Otherwise, run 'zpool status -x' to determine which pool has experienced a failure:

# zpool status -x
  pool: test
 state: DEGRADED
status: One or more devices could not be opened.  Sufficient replicas exist for
        the pool to continue functioning in a degraded state.
action: Attach the missing device and online it using 'zpool online'.
   see: https://zfsonlinux.org/msg/ZFS-8000-2Q
 scrub: none requested
config:

        NAME                  STATE     READ WRITE CKSUM
        test                  DEGRADED     0     0     0
          mirror              DEGRADED     0     0     0
            c0t0d0            ONLINE       0     0     0
            c0t0d1            FAULTED      0     0     0  cannot open

errors: No known data errors

Determine which device failed to open by looking for a FAULTED device with an additional 'cannot open' message. If this device has been inadvertently removed from the system, attach the device and bring it online with 'zpool online':

# zpool online test c0t0d1

If the device is no longer available, the device can be replaced using the 'zpool replace' command:

# zpool replace test c0t0d1 c0t0d2

If the device has been replaced by another disk in the same physical slot, then the device can be replaced using a single argument to the 'zpool replace' command:

# zpool replace test c0t0d1

Existing data will be resilvered to the new device. Once the resilvering completes, the device will be removed from the pool.

For an exported pool

If this error is encountered during a 'zpool import', it means that one of the devices is not attached to the system:

# zpool import
  pool: test
    id: 10121266328238932306
 state: DEGRADED
status: One or more devices are missing from the system.
action: The pool can be imported despite missing or damaged devices.  The
        fault tolerance of the pool may be compromised if imported.
   see: https://zfsonlinux.org/msg/ZFS-8000-2Q
config:

        test              DEGRADED
          mirror          DEGRADED
            c0t0d0        ONLINE
            c0t0d1        FAULTED   cannot open

Unlike when the pool is active on the system, the device cannot be replaced while the pool is exported. If the device can be attached to the system, attach the device and run 'zpool import' again.

Alternatively, the pool can be imported as-is, though it will be placed in the DEGRADED state due to a missing device. The device will be marked as UNAVAIL. Once the pool has been imported, the missing device can be replaced as described above.