Does the external drive have USB 2.0? Have you tried that interface? FW800 is the faster interface of course, but for troubleshooting purposes it would be helpful to know if it works with USB, or even FW400.
Not using Time Machine, can you copy a large amount of files to the drive using the Finder over FW800 without any issues?
Also, over the years Apple has released some firmware updates for some different G5 models, has yours been updated with any applicable firmware?
I have two OWC externals at work and I know one other person that also has two all without any issues. Of course, Time Machine wasn't being used in any of these cases. You might try sending an email to OWC support and see if they have any thoughts.
-Keith
P.S. Hey...we joined in the same year!
