New disk failed after sync. How do I recover?

ReadyNAS NV+

I had 3 1TB disks and was doing an upgrade to 3 2TB disks.

Over 2 days, I replaced a disk a night, letting it sync overnight.

Last night I replaced the final 1TB drive. Eight hours later the drive was synced. I chose to wait until the next night to reboot the NAS to do the expansion, knowing it would take several hours.

It had been working fine all day, but when I went to reboot using Raidar, I discovered this message:

Disk fail event occurred on SATA channel 3. If the failed disk is used in a RAID level 1, 5, or X-RAID volume, please note that volume is now unprotected, and an additional disk failure may render that volume dead.

I went ahead and rebooted but it never got past the Rebooting... message. So, I hotswapped the 3rd drive with the 1TB drive I had in there before. After a short while, I got this message:

Access to the disk on channel (??) is producing I/O errors. Although the array is still redundant, please replace this drive as soon as possible, as it is likely to fail soon

Now the logs say:

Sun Feb 24 23:40:16 CST 2013 RAID sync started on volume C. Sun Feb 24 23:41:45 CST 2013 System is up.

And the 3rd disk light is flashing but I can still access my shares. According to the Health screen, disk 3's status is "Resync".

What's going on and what should I do?

Thanks!