Could be a problem with the drive's RAM...not sure there's anything I can do about that, so I suppose it's time for a new external drive.
Just tested the situation with yet another external drive (a Seagate), and did not see the problem. So all indications seem to be pointing to problems with that one Toshiba drive.

How about I put "Solved" in the header?

I guess you could consider it solved, but I'm going to go buy a new drive today and test it out as well.
 
To JohnnyV:  That Toshiba drive is only about 45% full, and I'm not sure I'd be able to write or read anything to/from bad sectors, if bad sectors were the problem. I also believe the drive directory is in tact, since all files show up and can be read, and the metadata for each individual file seems to be uncorrupted, since all will open and function.
This 'problem' only seems to show up when writing .wav files to this one drive...go figure !

Bought a new external drive yesterday. So far, not seeing the problem occur at all, after several hours of backing up.
 
My guess now is that the original drive was going bad, due to a problem with the drive's RAM cache, as Anderton suggested earlier. It's interesting though, that this problem only happened with .wav files.
 
Anyway, thanks to Anderton, scook and JohnnyV for your comments and suggestions.