Have you tried inserting a different memory stick also? It might just be the formatting of your stick is incompatible, or even that it is simply broken. Two broken readers in a row sounds somewhat implausible.
Also, how do you figure the device does not even recognise the stick? Does it not show up in the cardreader slots when you connect the device to a PC / Mac, or is it a matter of not seeing the books stored on there? In the latter case, there have been similar reports of users with a PRS-700 who had to format their memory card and reupload their books to their PRS-600 before it recognised them.