Boot ISO files from USB with grub4dos

The goal is to make a universal bootable usb device with a small boot partition and a data partition on which we’ll store the iso files. This means you can just download almost any bootable iso and boot it without having to burn a cd or unpack the iso. We’ll install grub4dos as boot loader, using the ‘triple mbr’ feature to increase the compatibility with different mainboard and BIOS configurations. We’ll be using command line linux applications to reach our goal, any distro will do.

1) Find the path to your usb device, here we’ll use /dev/sdb
2) Optionally, to erase the usb device, issue the following commands as root:

shred -v -n0 -z /dev/sdb

3) Note down the size of the device in bytes:

fdisk -lu /dev/sdb

Disk /dev/sdb: 1031 MB, 1031798272 bytes

4) To make fdisk happy, partitions must end on cylinder boundaries. Here we’ll use the standard 63 sectors/track and 255 heads, so to calculate the number of cylinders and the last sector of the last cylinder, do the following:

num_cyl = floor( 1031798272 / 512 / 63 / 255 ) = 125
last_sector = num_cyl*255*63 - 1 = 2008124

5) We’ll make two partitions: a small boot partition with a ext2 filesystem and a second fat32 (or ntfs) partition to hold the iso files. Choosing ext2 as the boot filesystem hides it from windows systems, and makes the second fat32 partition visible. The boot partition only has to hold the ‘grldr’ and the ‘menu.lst’ file, so we set its size to 1 cylinder (or a little less if we take the first 95 boot sectors into account). The last sector of the first partition is 1*255*63-1 = 16064.

We use fdisk to set up the geometry of the device and make the first partition. Replace $num_cyl and $last_sector with the appropriate values.

fdisk /dev/sdb << EOF
x
s
63
h
255
c
$num_cyl
r
u
n
p
1
95
16064
n
p
2
16065
$last_sector
t
2
c
a
1
w
EOF

6) At this point, remove the usb device and plug it back in. The final layout should look like this:

fdisk -lu /dev/sdb

Disk /dev/sdb: 1031 MB, 1031798272 bytes
255 heads, 63 sectors/track, 125 cylinders, total 2015231 sectors
Units = sectors of 1 * 512 = 512 bytes
Disk identifier: 0x8d7751c5

   Device Boot      Start         End      Blocks   Id  System
/dev/sdb1   *          95       16064        7985  83  Linux
/dev/sdb2           16065     2008124      996030    c  W95 FAT32 (LBA)

7) Create the filesystems:

mke2fs -L boot /dev/sdb1
mkdosfs -F 32 -n data /dev/sdb2

8) As root, make a temporary working folder. Download the latest grub4dos and install boot code on the first partition:

cd
mkdir usbtemp
cd usbtemp
wget http://download.gna.org/grub4dos/grub4dos-0.4.4-2009-06-20.zip
unzip grub4dos-0.4.4-2009-06-20.zip
cd grub4dos-0.4.4
./bootlace.com --floppy=0 /dev/sdb1

9) Extract the first 96 sectors and create the triple MBR:

dd if=/dev/sdb of=boot.img bs=512 count=96
./bootlace.com boot.img
dd of=/dev/sdb if=boot.img bs=512 count=96

10) Mount the boot partition and copy the necessary grub4dos files:

mkdir /mnt/usbboot
mount /dev/sdb1 /mnt/usbboot/
cp grldr /mnt/usbboot/

11) As a test, we’ll boot the SliTaz LiveCD from the usb device. The iso will be saved to the second partition, and an entry in the ‘menu.lst’ file on the boot partition will make the iso available for booting.

mkdir /mnt/usbdata
mount /dev/sdb2 /mnt/usbdata
mkdir /mnt/usbdata/images
cd /mnt/usbdata/images
wget http://mirror.switch.ch/ftp/mirror/slitaz/iso/2.0/slitaz-2.0.iso
cd /mnt/usbboot/
umount /dev/sdb2
cat > menu.lst << EOF
title Boot SliTaz 2.0 LiveCD
find --set-root /images/slitaz-2.0.iso
map --mem /images/slitaz-2.0.iso (0xff)
map --hook
chainloader (0xff)
EOF
cd
umount /dev/sdb1

12) This should do it… To clean up just remove the ‘usbtemp’ folder.

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