"One or more drives is currently unavailable". That seems pretty clear: Try connecting the drives, turning them on, and verifying that they are visible. On a FreeBSD system, that could involve looking in dmesg for messages that the drives were recognized, looking in /dev/... for the device entries for the drives (both for whole drives, and then partitions/stripes by number and name), and by using gpart to see that the partitions are identified correctly and all there. On FreeNAS, I have no idea how to do this.
Anecdote: My wife used to teach the undergraduate physics lab. When students came to her saying that the equipment doesn't work, her standard question was: "Is it plugged in? Is it turned on?"