ZFS detach mirrored drives in a pool

I have an older (2-3 years) NAS running FreeNAS. I figured its time to upgrade some of the hard drives. Here's the current status: NAME STATE READ WRITE CKSUM Pool1 ONLINE 0 0 0 raidz1 ONLINE 0 0 0 ada3p2 ONLINE 0 0 0 ada4p2 ONLINE 0 0 0 ada5p2 ONLINE 0 0 0 mirror ONLINE 0 0 0 ada1p2 ONLINE 0 0 0 ada2p2 ONLINE 0 0 0 The drives in RaidZ1 are all 1GB and will stay. What's the best way to add 2 or 3 more 1GB hard drives to RaidZ1 and completely remove the mirrored drives, which are 500GB only, without: * Copying the data to a temporary holding spot * Recreating the pool * Copying it back The pool is about 50% full, so I'm not even sure there's data on the mirrored drives. How to check or where to begin?