Samsung Galaxy S3 does not support USB mass storage mode natively. Android 4.0 Ice Cream Sandwich seems to disable USB mass storage feature. Latest Samsung Galaxy S2 ICS firmware does not have this feature too. But worry no more, there is an application called SGS3 Easy UMS can help to enable USB mass storage on Samsung Galaxy S3. It will make external micro SD card on Samsung Galaxy S3 available as USB mass storage device. It supports exFAT for the moment (tried FAT32 but not working). Come on, try it out. Don’t stick on Media device (MTP) and Camera (PTP) mode only. Note – root access is required.