(Bootable) External USB flash drives can be booted via Grub4dos if this feature is supported by the BIOS. If the BIOS supports USB devices as HDD type, then the syntax would be in the (hd#) format – e.g. if the system has one local hard disk drive and a USB drive, and the local disk is set to boot before the USB flash drive, then the USB flash drive will be device (hd1), whilst the local disk will be device (hd0).