Initially I thought you were way off base, as this was happening with two new discs. However, I just turned them both over to look at them, and they're scratched to hell! It wasn't until after I tried copying the data in the Finder on the Mac, and noticed that it was taking 5+ minutes to copy as well. Very strange.
So it sounds like I need not worry about the SCSI errors that I'd been seening, and can use what you'd discussed in your previous post to "hide" the issue?