Harddrive failures? Maybe not

Question

Since the start of this year I've had a few, seeming, internal harddrive failures in my machine. It's a long story, but the details are important. You can scan my post history of the issues here:
http://discussions.apple.com/thread.jspa?threadID=1920757&tstart=0
Bottom line is that, before this latest incident, I went for over a month without any problems. On Sunday my machine froze and I couldn't restart from the internal. I booted from a LaCie firewire and tried to repair the drive with both DW and DU. Like the three previous times, implications were that my internal was toast. I couldn't erase or re-partition without:
- input/output error
or
- Disk could not be unmounted
On a whim last night, I pulled the internal (WD Caviar 320GB) and swapped it into one of my LaCie enclosures. It partitioned fine, so I took it another step and zeroed all data. That took about 4 - 5 hours, but it worked. I reinstalled Leopard, ran all updates and it worked fine. Note that on all previous failures this year, I never had problems booting off of my Firewire "Emergency" drive, just internals. Also, whenever I ran Techtool Pro 4 after each failure, every test came back "passed".
After searching the boards I thought that the issue could be related to the power supply, except if that were the case, wouldn't there be problems when booted from an external? FYI - My machine was one covered by an extended warranty because of defective power supplies and it was replaced by Apple. Possible thermal issues or a failing disk controller? I could just get another enclosure and not use the internal bus, but would like to understand the root of this problem. Sorry for the length of this post, but any help would be greatly appreciated.

Answer 1

Thanks for your response. I'm not actually using a UPS, just surge protectors and all peripherals are plugged into one. Good thought about the temp sensor. It had occurred to me with the second replacement drive and I made sure that it hadn't come loose on the board end of the cable. Vents have been cleaned out (and on a fairly regular basis...I'm a neat freak).
I appreciate other sets of eyes - making sure I'm not overlooking the obvious. When the original WD 80GB began acting up, I figured, no big deal, drives get old and fail. The second drive failure I blamed myself for being cheap and buying a refurbed WD 80GB. The third drive was a brand new WD 320GB and I was baffled because the symptoms were the same; an unexpected freeze, on restart it wouldn't boot; DW couldn't repair; DU couldn't reformat or repartition. I chalked it up to a defective drive and WD sent me a new one. But the replacement WD drive too?!
Here are the exact events leading up to this recent hd fu. Was editing a photo in Photoshop (two programs were opened, Firefox and Photoshop). I stopped to eat dinner and left the programs opened. Went back after dinner and the computer was frozen so I used the power button to shutdown, then restarted. I felt like I was in the movie Ground Hog Day! *Note that it's been almost a full day booting off of the drive since putting it in an enclosure, etc. External drives have their own power supply, so could this point to a failing iMac power supply (because it powers the internal bus)? I just can't imagine any crash trashing the Volume Header, catalogue, etc. so badly that I can't reformat.