I have a 4-drive RAID5 in Ubuntu 10.10 with mdadm. One of the disks seems to be problematic. It dropped from the RAID, and I re-added it and it was fine for a while, but now when I re-add it I get a FailSpare immediately. I checked the drive with DOS SeaTools, both long and short tests, and they passed. No SMART problems have been tripped. I ran the Ubuntu short self-test and it passed, but now after trying to re-add it, I can't read the drive's SMART data in Disk Utility, or run self-tests.
Is it possible this drive is defective even though it passes the self-tests, or is there something else wrong here?
Update: It turns out one of the SATA ports on my motherboard was likely defective. I replaced 2 (or was it 3) drives that (most probably) had no errors until I tried another SATA controller port and so far it works fine.