add a temporary MBR header to an NTFS partition to be passed to a windows VM

map_disk.sh

map_disk() {
	DISK=$1
	DM_NAME=$(basename $DISK)
	if [[ ! -e /dev/mapper/$DM_NAME ]]; then
		DISK_ID=${2-$(printf %s "$DM_NAME" | sha256sum)}
		DISK_TYPE=7 # NTFS
		MBR_FILE=$(mktemp -t vm-$DM_NAME-XXXXXXXX.mbr)
		BLOCK=$(basename $(readlink $DISK))
		BLOCK_START=$(cat /sys/class/block/$BLOCK/start)
		BLOCK_SIZE=$(cat /sys/class/block/$BLOCK/size)
		DISK_SIZE=$(((BLOCK_SIZE + 2048 + 2047) / 2048 * 2048))
		dd if=/dev/zero of=$MBR_FILE bs=512 count=2048 status=none

		LDEV=$(sudo losetup --show -f $MBR_FILE)
		END_GUARD=$((DISK_SIZE - 2048 - BLOCK_SIZE))
		if [[ $END_GUARD -ne 0 ]]; then
			END_GUARD="$((BLOCK_SIZE + 2048)) $END_GUARD zero"
		else
			END_GUARD=
		fi
		sudo dmsetup create $DM_NAME <<EOF
0 2048 linear $LDEV 0
2048 $BLOCK_SIZE linear $DISK 0
$END_GUARD
EOF
		CYL=$((DISK_SIZE / 2048))
		if [[ $CYL -gt 65535 ]]; then
			CYL=65535
		fi
		sudo dmsetup setgeometry $DM_NAME $CYL 64 32 0
		sudo chown "$USER" /dev/mapper/$DM_NAME
		sfdisk --quiet --no-tell-kernel --no-reread /dev/mapper/$DM_NAME <<EOF
label: dos
label-id: 0x${DISK_ID:8}
device: $DM_NAME
unit: sectors
grain: 512
sector-size: 512

${DM_NAME}1 : start=2048, size=$BLOCK_SIZE, type=$DISK_TYPE
EOF
	fi
}