sorry for the inconvinience..
yes i do encounter the same problem with a class 4 16 GB Sandisk mSD card...
did the format on another samsung phone, tried the format via Windows, but still
if you choose to have the SD card as an internal storage device, the M5 warns you that the card is too slow and it might not make the best of tit and that some apps will boot up slowly or will not respond at all (did`t quite spelled that but in sense - you got the point!?)
and then after I have installed one app and everything goes OK, the phone sudenlly restarts at its own...and offcourse wont boot up to Android...
Many thanx for your help
I found out that if you choose the mSd card as a remowable storage unit, the phone works just fine!
;D