Thanks Joe,
I checked the cables--everything seemed fine. I then re-slotted the drives (my server uses the IcyDock enclosures) just to make sure that wasn't an issue. I then powered up the system--I heard the clicking noise indicative of bad drive. When I went to the unRAID web console, it was showing an odd message. Disk 3 had a red dot and was marked as unavailable, but the error message was that the parity disk was smaller than the largest disk in the array. Disk 3 was showing a total size of over 2.1GB, even though it's a 1.5GB drive (my parity is 2GB Hitachi). I then unassigned disk 3, and was able to successfully start the array. I then tried to use preclear on disk 3, and got the message "Sorry: Device /dev/sdn is not responding to an fdisk -l /dev/sdn command", so I have to assume the drive is beyond recovery at this point. I guess it's another RMA for Seagate.
I was still a little concerned about disk 1, so I ran a SMART check on it. Attached is the output; based on your wiki post about interpreting the SMART parameters, this drive looks like it may be on the road to failure, correct?
smart_sdm.txt