I have a pretty serious problem with my Dell Inspiron 600m laptop running Win XP SP3. It seems that any USB thumb drive that I plug into it is and plug into my Win 7 box is no longer a valid drive according to Win 7. I can copy files to it on the laptop, but Win 7 then says it isn't a valid drive. However it is still readable by the laptop. Win 7 can't even format it since it says it isn't a valid drive. The drive worked just fine in Win 7 immediately before I plugged it into the laptop. I tried this twice with 2 different thumb drives; the results were the same (a Lexar and a Cruzer).