Running S.M.A.R.T. Tests | TrueNAS Documentation Hub, I suggest weekly long tests and daily short ones, plus running @joeschmuck’s Multi-Report.
Kinda: in a MIRRORED VDEV you can always add or remove drives[1], thus increasing or lowering the parity of the VDEV; the only way of increasing a MIRROR VDEV available space is by replacing all the drives of that VDEV with larger ones.
Glad to give back what this community gave (and still gives) me!
You cannot remove the last drive of a VDEV if there is not enough free space in the other VDEVs to host the “wannabe removed” VDEV’s data. ↩︎