The only solution that I found worked at my end (and I really combed through and tried all of the suggestions) was to do what I described above -- repairing permissions with Disk Utility, Restarting, Downloading.

What would you suggest for those users that do not have Disk Copy? I don't know when Apple made the change, but mounting images appears to be handled by the Disk Utility (and I tried using those features therein as well). I haven't seen disk copy since Panther (10.3) if not earlier. And Disk Copy can not be downloaded from Apple, as far as I can tell. But it can be kind of hard to find stuff there sometimes.