Sony specifically states that you should not format the memory stick duo while in windows,that you should only use the PSP to format the mem stick.....
But,try this: 1.) Make a folder on your hard drive and call it PSP
2.) Inside that folder make these folders : SAVEDATA
this is the file structure originally on the PSP Mem stick.
If you format the mem stick duo or delete any thing off of it using windows ,the PSP won't recognize that is been formatted or that you've deleted anything off of it,you have to use the PSP to properly format the stick,so I think that your PSP can't read the memstick because the original file structure isn't there anymore or it thinks the memstick is full?
3.) Make sure that your memstick is completely empty
by reading it in windows,then send that folder structure
that you made in step 2 to the PSP.
then try reading it in your PSP again.....also to play videos you need another folder on the PSP mem stick called MP_ROOT,then inside of that folder a folder called 100MNV01.....They show you exactly how to do it on psp.connect.com......i don't know if this will get your PSP reading you memstick,but it might help,also the memstick is formatted in FAT