portable external hd, W7 wants to format,but ok in XP, anybody know why

i decided to move most of the 'movies' i got on my laptop (W7, 64bit) to an external portable hd, this was done with no problems, a few weeks later i decided to plug this hd back into my laptop and check which movies i had, the hd opened i could see the movies so i clicked on one to be played but i got a message saying 'file not found' so i decided to plug the hd into a different usb port on the laptop, this time got a message saying (something like) 'device needs formatting' strange i thought..

Anyway i had an idea to plug this portable hd into my XP desktop, the hd opened and i could play all the movies with no problems, this is baffling me, can anybody explain why this happened.....thanks.