I face the exact same issue and came here searching. X10 mini pro. switching from 2 to 8 gb. i tried formatting from phone, copying content etc. but to no vail.
Infact i kept a blank 8gb card (formatted within the phone) in the phone for a while. still it freezes and stuff.
Moreover the phone belongs to my wife... so i face more "difficulties"
please help my poor soul :P
EDIT: Found this Main: Problem using 8GB MICRO SDHC card with x10 mini pro
Apparently class 4 cards have an issue with the X10 mini pro.
class 2 or class 6 is OK. but not 4