VBoxManage clonehd d6b9f0a5-98df-48ca-83c8-91a0809ec349 --format RAW Debian.raw
sudo dd if=~/.VirtualBox/HardDisks/Debian.raw of=/dev/sda5

EXT4-fs (sda5): VFS: Can't find ext4 filesystem

losetup -fs Debian.raw     # prints /dev/loop0 (or some other number)
mount /dev/loop0p1 /mnt    # 0 as above, 1 is the partition number

fdisk -lu Debian.raw  # note starting sector $S
losetup -fs -o $(($S * 512)) Debian.raw
mount /dev/loop0 /mnt  # /dev/loop0 or whatever losetup prints

<Debian.raw tail -c +$((512*$S)) | dd of=/dev/sda5 bs=4M

losetup -P -f --show my.img

$ los my.img
/dev/loop0
/mnt/loop0p1
/mnt/loop0p2

$ ls /mnt/loop0p1
/whatever
/files
/youhave
/there

$ sudo losetup -l
NAME       SIZELIMIT OFFSET AUTOCLEAR RO BACK-FILE                                                                                      DIO
/dev/loop1         0      0         0  0 /full/path/to/my.img

$ # Cleanup.
$ losd 0
$ ls /mnt/loop0p1
$ ls /dev | grep loop0
loop0

los() (
  img="$1"
  dev="$(sudo losetup --show -f -P "$img")"
  echo "$dev"
  for part in "$dev"?*; do
    if [ "$part" = "${dev}p*" ]; then
      part="${dev}"
    fi
    dst="/mnt/$(basename "$part")"
    echo "$dst"
    sudo mkdir -p "$dst"
    sudo mount "$part" "$dst"
  done
)
losd() (
  dev="/dev/loop$1"
  for part in "$dev"?*; do
    if [ "$part" = "${dev}p*" ]; then
      part="${dev}"
    fi
    dst="/mnt/$(basename "$part")"
    sudo umount "$dst"
  done
  sudo losetup -d "$dev"
)

sudo apt-get install kpartx
losetup -fs my.raw
sudo kpartx -a my.img
ls /dev/mapper

/dev/mapper/loop0
/dev/mapper/loop0p1

mkdir -p d
sudo mount /dev/mapper/loop0p1 d