I have 4 disks that are currently :-

320GB macos boot disk
320GB time machine
500GB mirrored data
500GB mirrored data

Running out of space on my mirrored data volume, so just upgrading it with a pair of 1.5TB drives, and so a little bit of a shell game… well I’m cheating and using ZFS as well, so here’s what I’m doing:-

$ diskutil disk1
$ sudo zpool replace disk0s2 disk1s2

wait for resilver to complete

greebo:~ mike$ zpool status
pool: pool1
state: ONLINE
status: One or more devices is currently being resilvered. The pool will
continue to function, possibly in a degraded state.
action: Wait for the resilver to complete.
scrub: resilver in progress, 20.39% done, 3h9m to go
config:

NAME STATE READ WRITE CKSUM
pool1 ONLINE 0 0 0
mirror ONLINE 0 0 0
disk2s2 ONLINE 0 0 0
replacing ONLINE 0 0 0
disk0s2 ONLINE 0 0 0
disk1s2 ONLINE 0 0 0


Move boot disk onto newly freed up drive using ASR

greebo:~ mike$ sudo asr restore --source / --target /Volumes/greebo/
Password:
Validating target...done
Validating source...done
Validating sizes...done
Copying ....10....20....30....40....50....60....70....80....90


swap original boot drive (320GB) with a new 1.5TB drive and then repeat the zpool replace command

sudo zpool replace disk2s2 disk3s2

wait for the final resilver to complete and then export/import the pool to grow it.


$ sudo zpool export pool1
$ sudo zpool import pool1


Now we have spare space!

# zpool list
NAME SIZE USED AVAIL CAP HEALTH ALTROOT
pool1 1.36T 407G 989G 29% ONLINE -