From MS, the autorun capabilities are restricted to CD-ROM drives and fixed disk drives. If you need to make a USB storage device perform Autorun, the device must not be marked as a removable media device and the device must contain an Autorun.inf file and a startup application.
The removable media device setting is a flag contained within the SCSI Inquiry Data response to the SCSI Inquiry command. Bit 7 of byte 1 (indexed from 0) is the Removable Media Bit (RMB). A RMB set to zero indicates that the device is not a removable media device. A RMB of one indicates that the device is a removable media device. Drivers obtain this information by using the StorageDeviceProperty request.
Check this out.