余ってるHDD適当に置き換えるつもりが放置プレイしてた。8Tくらいのに置き換えてみるか。

pool: zfs
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: http://www.sun.com/msg/ZFS-8000-2Q
scrub: scrub completed after 13h51m with 0 errors on Sat Jan 21 17:34:55 2017
config:

NAME STATE READ WRITE CKSUM
zfs DEGRADED 0 0 0
mirror-0 DEGRADED 0 0 0

errors: No known data errors