Tienes ICS instalada? Prueba esto:
1) Download/install
Android Terminal Emulator, open it and type:
su (to grant superuser permission)
then
setprop persist.sys.usb.config mass_storage,adb and execute the command 2) and Reboot phone typing
reboot
3) If it is still not working grant yourself su rights by typing
su - but now type
setprop persist.sys.usb.config mtp,adb and
reboot