Grunt, that did it!
Basically, it came down to needing to upgrade the NVidia drivers for the SATA controller, but doing it the normal way was useless as it kept telling me that the drivers WERE up to date.
Going directly to NVidia's site and downloading an "update package" took care of it, and a drive just formatted to 100%! I'm going to do a "check disk" on it just to be sure, then do the same process for the second, replacement disk (I'll just keep it, as WD charged $30 less than the good price I paid for the first one!).
So, thanks to everyone... many of you were right about the SATA controller needing updating. Special thanks to Grunt for pointing out that I can't do that update in the normal way.
Tom was right, you guys ARE awesome!