funny you should post this. i was experiencing the EXACT same problem only yesterday. i got myself a 2GB mem stick (duo pro). popped it in, and it registered that is was there.
as i wanted to copy all the music and pics from the mem card that came with it, i put the 2gig into the adapter and into my card reader. plugged the usb phone cable into my pc and copied everything across.
put the new mem card into the phone, pressed the little info button on the side of the phone.. it was reading the mem stick, played a few tracks and everything is fine. add more songs with disc to phone, play them fine.
a couple of hours later, all i can getis what is saved on the phone. so i tried to format, but nothing. bizzarre.
this is the bit you actually need to read (ive been waffling)
put the old mem card in to check if that is being read, nothing. now i really start panicing.
took out the cars and blew into the slot where it sits, and still nothing. plugged it back into the pc, and it read it ok. swapped the cards back over, and everything is fine.
(oh, dont know if you are in the same boat but i thought that this might of been the reason. i have the k750i and flashed the firmware to w800i) what matters though is that it is working now though.
hope you get your sorted.