So I'm testing this out in VirtualBox, and I've got two 10GB drives as a test (mirror of ada1 and ada2, ZFS). I create a zpool/volume called "tank" with the two drives, no problem. Then I power off, remove one drive (ada2) and boot back up. It says "DEGRADED" which is expected. I shut down, add a brand new 10GB HD and boot up again. Using the GUI, I list the disks in the volume and for ada2 I click "Replace" and choose the only option available (in-place). It seems to work, as I go into the shell and type "zpool status" and it says it resilvered it, but it still says DEGRADED even after scrub & clear. Any ideas?
Got it. Switched to FreeNAS 7.2 which seems more stable, and also lets you handle HD replacement via the GUI. The solution was to offline the drive, shut down, replace the drive, boot up, and do a zpool replace.