Recommended way to add a disk to a VDEV

[Davvo]

@milkywayfarer

Step 2, but do not create a RAIDZ1 and make a RAID5 array that is expandable.

Using hardware RAID with ZFS is a sure way to lose data.

Another option is to take the new drive and one of the 3 in RAIDZ1 and create a new pool with a single mirrored VDEV, then use local replication to copy the old pool in the new, finally destroy the old pool and add the two drives as a second mirrored VDEV to the new pool. But you need enough space.

I strongly suggest reading Introduction to ZFS | TrueNAS Community and iX's ZFS Pool Layout White Paper.

Planning is a necessity with ZFS.