Is this the new drive, or the original one? If you've switched out the drive itself, I'd expect the case is the problem.

It's an old (two or three years?) Western Digital 150G Firewire. It came in an enclosure/case and I haven't touched it or made any modifications. So it's the original one.

OK. Well... I think it might be time to retire it, or at least replace the drive in the mechanism to see if it's the drive or case.

Same Problem here
 

I had this exact same problem. But mine showed an error on different files each time that I tried a backup. I tested each of the files that had errors and they were all fine. I even replaced them with backups from other drives just for safety sake. And they would error out again, although sometimes it would be different files again. None of the error listed files were actually bad. They were all fine. I see a ton of messages in my syslog when running SuperDuper, though. I get this error:

/Applications/SuperDuper!.app/Contents/MacOS/SDCopy: error processing extended attributes: Cannot allocate memory

Hundreds of messages in the log. Maybe you can help out with what that is all about.

Anyway, I ended up sending my external drive to the manufacturer and they told me that there was nothing wrong with the drive but they replaced it for me anyway. So I'm running SuperDuper again right now on my drive (haven't changed anything on my internal source drive. So far, so good. It's copying but I'm getting the syslog errors. Also, the copies worked fine for a couple of months just like yours and then they just stopped with the I/O errors. I'm wondering if there is a bug in the code somewhere that's causes the error. Very weird that it's happening. I love the program and REALLY would like to get this fixed. But I'm concerned that the errors will start up again. Thanks for your help Dave. Hope we can get it fixed.

The syslog error is known, and is discussed elsewhere on the forums. It's a diagnostic produced by an OSX API, at a warning level.

We've figured out a way to work around the problem, and there'll be a fix in the next update of SD. It doesn't have anything to do with the errors, which were very likely caused by a bad destination drive. (As described in the Troubleshooting section, when it moves around, or the file on the source is OK, it's a problem with the destination.)

Hope that helps...

Well, for mine at least, according to the manufacturer, my drive was fine. They could find no issues whatever with the drive. But since I insisted, they replaced it anyway. I'll let you know if the I/O errors start up again. Thanks.

Well -- it wasn't, though. Otherwise, the problems would have continued with the new drive! :)