Bizarre Time Machine failure/error - iPhoto, Firewire?

I'm experiencing a Time Machine backup error & I've been able to recreate it reliably but it seems very strange. Here's what's going on:

- TM used to work fine on that iMac; only recently have the problems surfaced (not sure if it coincides with 10.5.2 upgrade but it might)
- Now, when I try to use TM, I get a standard error (TM failed...). Upon checking the logs it appears to fail while coping a portion of the iPhoto Library ( /Thumb32Segment.data: Operation could not be completed. OSStatus error -36).

Here's what I've done so far:

- Wiped & reformatted the external HD (which uses Firewire 400). GUID partition map
- Rebuilt the iPhoto Library (which is about 10 GB in size)
- I've also had problems simply copying the iPhoto Lib to the external HD (direct copy failed); however, I have no issue opening iPhoto & using it, making a copy (duplicate) on the internal (system) disk or copying it across the network to another Mac.
- I've run Disk Utility and TechTools Pro (current version) and found no problems with any files.
- The error copying iPhoto to the external HD using the Finder indicated that the Firewire bus was reset xx times prior to the failure (and it seemed like most of the data was copied over before it failed).

I will do a few more tests - like using TM to backup to a different external HD, running Disk Warrior, testing the first external HD with another Mac using TM (to see if there's a problem with the first external HD), etc. and report on what I find.

If anyone has any suggestions or ideas, please let me know.

Thanks!