JC's instructions above might should also state in #3: After you have copied (or moved) all the contents OFF the Media Card to your PC, delete all the files on the Media Card. You are making sure all the "BBthumbs.dat" files and everything else on the card are removed from the card. It is the "BBthumbs.dat" that seems to be the culprit on the card, as those files were created with OS 4.2, and don't seem to be compatible with 4.5.
So you are placing the card back in your BB as a blank or empty card. Then continue with formatting the card in the BB with OS 4.5 running.