gimp wrote:0. Safe to say one or more drive is bad?
Excellent troubleshooting, btw. That would be my guess.
gimp wrote:1. Should I use seatools to check each drive? I don't have any way to do this.
Yes, you can use Seatools. If you have access to a PC with SATA drives, you can burn Seatools to CD-ROM, connect each disk (one-at-a-time) and then boot the system from CD. You should be able to run all the tests (I recently did this to test a bunch of used drives I purchased).
gimp wrote:2. Does using seatools destroy data?
No, the tests are non-destructive.
gimp wrote:3. Could this be a drive firmware issue?
Possibly, but if the system was working fine previously, I would tend to think memory or hard drive. You have already run the memory tests and have isolated each disk, which seems to lead to disk #2 as the trouble.
gimp wrote:4. Should I just do a warranty exchange on drive 2 and hope for the best?
Yes. RMA the disk and keep it as a cold spare in case something like this happens again.
gimp wrote:5. Could I replace drive 2 with the new one I bought?
Yes. The 333 is the replacement for the 340 (it has 3 platters now, rather than 4).
gimp wrote:What are the implications that the new drive has been formatted/initialized as the only drive in the NAS?
Will the system boot with just disks 1, 3 & 4 installed? If so, hot-add
the new 333 disk and it should initialize & re-sync the data. In about 10 hours you should be back in business.
Good luck and keep us posted.