# Say I have a clean external harddrive connected to a working linux machine,
# Now I want to install debian on it so it boots when I insert it into some other computer.
# Say the harddrive is connected as /dev/sdx. All data on it may be wiped.

# Create partitions on that harddrive.
# You can use `sudo cfdisk /dev/sdx` or `gksudo gnome-disks` for pointy-clicky.

# Say we now have partion /dev/sdx1 created to become the new root filesystem.
hd=/dev/sdx
root_partition=/dev/sdx1

# Create file system:
mkfs.ext4 $root_partition

# mount
mount_at=/mnt
mount $root_partition $mount_at

# Install debian. Let's say testing. This may take a minute.
debootstrap testing $mount_at http://deb.debian.org/debian/

# make sure special magic linux stuff exists and chroot
for i in dev dev/pts proc sys run; do mount -B "/$i" "${mount_at}/$i"; done
chroot $mount_at bash

# the chroot forgot above vars...
hd=/dev/sdx
root_partition=/dev/sdx1

# The new system has no kernel yet.
apt-get install linux-image-amd64

# The new system has no fstab yet. Assuming that upon boot, the new root disk
# will be /dev/sda1, we could use that. Probably safer would be to use the
# uuid:
uuid="$(dumpe2fs $root_partition | grep UUID | sed 's/.*:[ \t]*//')"
echo $uuid

cat /etc/fstab
# rm /etc/fstab
cat >> /etc/fstab <<END
UUID=$uuid / ext4 defaults 0 1
proc /proc proc defaults 0 0
END

# bootloader.
apt-get install grub2 vim
# installer may ask whether to continue without installing grub, say yes.
grub-install $hd

# Tell grub what to boot ... when it finds itself in the other computer!
update-grub

# The default entry should actually work out despite all the wrong hd numbers,
# I guess from that 'search' for the UUID.

# If it doesn't, maybe populate a custom boot entry.
# When in the other computer, this hard drive will likely be the first:
hdnr=0
# If it is the first partition, this should be 1:
hdpart=1
# kernel files...
vmlinuz="$(ls -1 /boot/vmlinuz*)"
initrd="$(ls -1 /boot/initrd*)"

cat > /etc/grub.d/40_custom <<END
#!/bin/sh
exec tail -n +3 \$0

menuentry 'New system' {
        insmod part_msdos
        insmod ext2
        set root='hd${hdnr},msdos${hdpart}'
        search --fs-uuid --set=root $uuid
        linux ${vmlinuz} root=UUID=${uuid} ro net.ifnames=0 biosdevname=0 quiet
        initrd ${initrd}
}
END

# generate the grub.cfg including above entry
update-grub

#less /boot/grub/grub.cfg

# root password
passwd
#secret
#secret

# misc stuff to install...
apt-get install locales-all
apt-get install xorg xserver-xorg-input-all xdm ...

# done in chroot
exit

# recursively unmount
umount -R $mount_at
sync

# insert HD into other computer and boot.

# From the new system, re-update grub to generate a proper menu for that system.
update-grub