A Method for Creating a Bootable Live Linux Flash Drive



Version 0.6
Copyright © 2007,2008 by Zack Smith

Introduction

Many technically-inclined people have formed a keen interest in booting from flash. It is indeed an appealing idea, perhaps due to:
  1. The fact that many computers (even relatively old ones) are bootable from flash.
  2. The fact that flash thumbdrives are now commonplace.
  3. The cheapness and large capacity of such drives.
  4. Possession of unused or underused older-generation flash cards such as CompactFlash that are no longer useful in most cameras.
  5. A geeky desire to experiment with tech gear.
  6. A geeky desire to run Linux on every concieveable computer.

There are many pages on the Web that offer tantalizing clues on how to create a bootable flash drive, however none of them describes the procedure with clear, step-by-step instructions. Many actually link to other pages effectively saying "he knows better than me", or "use that utility program", but these other pages typically either don't exist or offer no real explanation either, or the utilities don't run properly.

I created this page to explain, once and for all, how to create a bootable live Linux flash drive in clear steps, and not merely a flash-based Linux installation drive. This drive is intended to supercede the hard drive, either for the purpose of recovery or to replace it.

Equipment

Target (boot) system

I will be booting the flash drive in a 150 MHz Thinkpad 560e laptop. It's the perfect notebook for taking to a cafe or library because it cost me $30 used, although it lacks a hard drive. I am booting from a tiny 48MB CompactFlash card that is in a 16-bit PCMCIA adapter, which is plugged into a The 560e's BIOS permits booting from a flash card in the PCMCIA slot. This laptop has no USB ports.

Host system

My host system for preparing the flash drive is a desktop that is running Slackware 12 (Linux 2.6 kernel). I've placed the 48 MB CompactFlash card in a USB 2.0 CompactFlash adapter, which appears as /dev/sda.

The Steps

You will need to log in as root.

1. Unmount the flash card if necessary.

If you previously modified /etc/fstab to automount /dev/sda, type: 
umount /dev/sda

2. Wipe the flash card.

Let's start clean. Remove any useful data from the flash card first because we will wipe it using this command: 
dd if=/dev/zero of=/dev/sda

3. Partition the flash card.

We need to create two partitions. The boot partition and the root partition, so: 
fdisk /dev/sda
(You can also use cfdisk if you prefer a simpler user interface.) Fdisk may start up by complaining about the boot sector, because it's all zeroes. That's fine. Create a primary 4 megabyte partition using 'n'. Use the 't' command to set it to type 1, which is FAT12. Set it to bootable using the 'a' command. Then, create a second primary partition (number 2) using 'n' and leave its type alone since the default is Linux ext2/ext3 and that's what we want. Type 'w' to write the partition information to the flash disk.

4. Format the partitions. 

mkdosfs -F12 /dev/sda1 

mkfs.ext3 /dev/sda2

5. Create the boot sector. 

lilo -M /dev/sda
LILO will tell you that it backed up the original boot sector in /boot. Since we had wiped the disk previously, that sector is not useful; you can delete it.

6. Install the SYSLINUX bootloader.

It simply works. 
syslinux /dev/sda1

7. Find or create a suitable kernel.

Depending on which processor your target system has, you may not be able to use your host system's kernel. For instance, my host is a Pentium III, and I had previously created a kernel compiled just for that CPU. The stock kernel was for a Pentium Pro, I believe. Both are too new for my laptop's Pentium I MMX.

If you've never compiled your own kernel, now is not the time to be a wuss! Creating a kernel is no more difficult than making coffee over a campfire. Here is what you do, using the example of creating kernel 2.6.20.21, which is more reliable than 2.6.23. 

cd /usr/src 
ftp ftp.kernel.org 
   cd pub/linux/kernel/v2.6 
   bin 
   mget linux-2.6.20.21*bz2  
   bye 
tar jxfv linux-2.6.20.21*bz2 
cd linux-2.6.20.21 
make menuconfig
...Whereupon you will look at all the curious and interesting options that you can select or not. Examine all the options' details for your edification and enrichment.

The goal should be to create a small kernel that has all the drivers you will need, and none that you don't. If the kernel contains all the drivers, there will be no need for an initrd (initial RAM disk), whose purpose is to provide drivers.

Once you have a nice set of options, exit, saving the configuration, and then type "make". When the compilation is complete, your kernel will be arch/i386/boot/bzImage.

In my case, I clearly need support for PCMCIA for the flash and I need to support the older non-SMP CPU and perhaps APM power management. My target system doesn't support AGP, USB, Firewire, framebuffer graphics, Ethernet, wireless and the like.

8. Copy the suitable kernel into /dev/sda1

In my case, I have a directory /usb already created that I use for accessing USB drives. 
mount /dev/sda1 /usb 

cp /usr/src/linux-2.6.*/arch/i386/boot/bzImage /usb/bzimage.i

9. Create a SYSLINUX config file 

vi /usb/syslinux.cfg
Here is mine. Note that I am telling SYSLINUX that the root filesystem is in /dev/hdc2, which is the PCMCIA IDE drive, i.e. the CompactFlash card. Partition 2 (hdc2) is of course the Linux ext3 filesystem. 
default bzimage.i

prompt 1

timeout 1200

display message.txt

F1 message.txt

label bzimage.i

  kernel bzimage.i

  append vga=6 root=/dev/hdc2 load_ramdisk=0 ro SLACK_KERNEL=bimage.i

10. Install Linux

This is the part that could require the most time.

You need to have your Linux installation DVD or CD handy. Plug the flash card into your host computer and boot the host with the Linux install disk in the optical drive. Select your target to install onto as simply the same /dev/sda2 as above. In the case of Slackware, which I use, they ask what the target drive is and the installation disc's kernel supports USB flash adapters.

What takes time is selecting which specific packages (e.g. RPMs) to put into the 2nd flash partition. Unless your flash card is 4 gigs or more, you will want to save any megabytes you can. In the case of my 48MB flash disk, very little could fit except Slackware's A series packages and a few important libraries.

I later used a 1 GB CompactFlash to install important parts of Slackware's A, AP, D, L, X and XAP series in about 700 megabytes. I specifically left out junk that I don't need, like the GNU Ada compiler, Common Lisp, tape drive programs, CD/DVD software (my system has no optical drive) and the like. 

From this experience, I believe that a 2 gig flash drive
would be ideal, 4 gigs would allow for media files
enough to entertain, or if you have serious work to do, A-DATA makes a
16 gig CompactFlash card that costs about $80 via Newegg.

11. Tweaking

After installing Linux you will have to edit your /etc/fstab, since the installation process will assume that whatever your flash card was called on the host, that's the same as what it will be called on the target, which is probably not true.

12. Testing

Now boot from flash and see how it works. It will be a little strange to use a computer with no hard drive noise. Still, it works great, even in my case using a 16-bit adapter with 700 kB/sec throughput. I've set up both a 48 MB rescue disk and a 1 GB everyday use disk successfully.

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