特殊:Badtitle/NS100:BootFromUSB

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This page explains how to boot an Ubuntu system which is installed on a USB device when the computer does not support USB booting

Introduction

If you want to install Ubuntu onto a USB drive, such as a large "USB stick" or an external hard drive it is often impossible to boot this system on slightly older computers. This guide explains how to create a bootable CD which will load the USB system. Some recent computers lack the BIOS option (or menu entry) to boot from a USB drive, but they are often capable of detecting the USB drive, in which case grub can be used to boot the drive.

How It Works

PCs use a system called the BIOS (Basic Input/Output System) to start the machine. Even though a few features have been added to many BIOSs over the last 25 years the BIOS system is still extremely limited, as it is part of the "bootup" procedure (so called because the computer has to "pull itself up by its boot straps", a feat which is impossible in real life but aptly describes how a simple electrical current supplied with a power switch can start a chain reaction of activating ever more complex electronics until the computer system is up and running. The BIOS is key to this, so it cannot be very complex). There have been several alternatives for the BIOS system such as the Amiga Kickstart, which uses up to half a megabyte of ROM chips to store large parts of the operating system (although this makes booting Linux on an Amiga slightly more difficult since we do not want the AmigaOS which is in the Kickstart chips) and more recently the EFI system used in Intel based Apple Macs, but for now the PC system is stuck with the BIOS, and this guide is for those of you who's "extra features" don't extend to USB support. So, how do we get around these inherent limitations in the computer's boot system? The answer is that we can't. Instead we must add another layer to the bootup procedure by adding a piece of software which the BIOS can access, and which in turn can access the USB drive for us. Luckily there is already an excellent piece of software which can understand USB devices and its name is Linux. As long as there is a Linux kernel on a device which the BIOS can understand then we can let this Linux kernel take over to do what we want. Additionally, in some cases the grub boot loader can do the job directly, without the need of a linux kernel. A stripped down Linux kernel will fit onto a high density (1.44MB) floppy disk, but the standard Ubuntu kernel doesn't quite. Also, the standard Ubuntu kernel does not have all of the drivers needed to boot a USB drive, thus these extra driver modules must be put into the "initial RAM disk" image (abbreviated to "initrd") which gets copied into the RAM during boot to enable the kernel to access the extra modules it contains. Most PCs can boot from a CDROM drive, and writable CDs can store more than enough data to allow Linux and an initrd to fit comfortably (that's how "live CDs" work) so we will use this method to boot. Since the kernel and initrd get copied into RAM during boot and run from there (the RAM based initrd is not needed once the kernel has access to the main drive anyway) it is not necessary for the CD to be in the drive after booting has finished, so if you only have one CDROM drive it will not be tied up like it would with a live CD. So now we know how Ubuntu can be booted from CD into a USB drive in theory, it is time to put it into practice.

Booting via grub

This is the easiest way, and has been proven to work on several laptops from 2002-2005. To check if your BIOS is able to detect the USB drive and hand it over to grub, just run grub (from your hard drive if it already installed, or from a grub boot floppy or CD). At the grub menu, hit 'c' to enter command mode. Now search for your USB drive, using the root command to choose a drive/partition and the find command to see if you found the right one. You can go through your devices like in this example:

grub> root (hd0,0)   # first harddrive, first partition
grub> find /[tab]    # type the slash then press [tab], and it will try to list files on this partition
Error 17: Cannot mount selected partition   # Oops no file system here
grub> root (hd0,1)   # first harddrive, second partition
grub> find /[tab]
Possible files are: lost+found var etc media ...   # That was my hard drive with my linux install
grub> root (hd1,0)   # second hard drive usually is the USB drive if you have only one internal drive
grub> find /[tab]
Possible files are: ldlinux.sys mydoc myfile mystick syslinux.cfg  # Bingo, that's the USB stick

(If you have 2 internal drives, the USB drive probably is hd2,0 and so on.) Or, if you know that there is a file called e.g. "MYDRIVE" on the drive, just run

grub> find /MYDRIVE

and grub will look through all drives and partitions that it can access. It will list the correct device to use for the root command. Boot the drive by entering:

chainloader +1
boot

For convenience, add these commands to your grub configuration (usually in /boot/grub/menu.lst):

# to boot from a usb device
title    Boot USB drive
root     (hd1,0)
chainloader +1
boot

Of course, if you don't have grub installed on your hard drive, change the menu.lst on your grub floppy or CD. If you are not able to find the drive with the help of grub, you have to use the linux kernel as explained in the next section.

Booting the kernel from a bootable CD

You can install Ubuntu using a regular CD or DVD, since you will be booting from discs which contain a Linux kernel and all of the driver modules needed to acces your USB drive. The only difference you may notice is that your drive will be called "/dev/sda" instead of the regular "/dev/hda" which you may find in many guides. Once the installation has finished using the disc you must reboot into your new USB system. This is where you need a bootable CD. It is easiest to make such a CD from another Ubuntu system, slightly harder from a different Linux system, and quite hard on a Windows system. It is usually easiest to boot from an Ubuntu Live CD. To build your own boot CD you will need to mount your system from within the Live CD (this is describe here UbuntuHelp:Mount). Once that is done it is time to add the needed modules to your initrd. To do this you must run: On Dapper (6.06LTS):

gksudo gedit /wherever_you_have_mounted_your_system/etc/mkinitramfs/modules

On Edgy (6.10) and Feisty (7.04):

gksudo gedit /wherever_you_have_mounted_your_system/etc/initramfs-tools/modules

Which will open the text editor with that file. You need to add these lines to the end of the file:

usbcore
sd_mod
ehci_hcd
uhci_hcd
ohci_hcd
usb-storage
scsi_mod

(It is always a good idea to put comments in any manually changed configurations so you don't get confused later) Now save the file and exit gedit. There is one small problem before we carry on. USB drives take a few seconds before they are set up properly by Linux, and your boot would fail before your drive becomes accessible. To sort this we need to tell the initrd to wait a few seconds before it gets carried away and fails. You can do this by running: On Dapper (6.06LTS):

gksudo gedit /wherever_you_have_mounted_your_system/etc/mkinitramfs/initramfs.conf

On Edgy (6.10) and Feisty (7.04):

gksudo gedit /wherever_you_have_mounted_your_system/etc/initramfs-tools/initramfs.conf

Now to this you should add at the very top the lines:

WAIT=15

Then save the file and exit the text editor. Now that we have corrected the initrd's setup we must use this setup to rebuild the initrd using our new guidelines. To do this you must enter your system by running:

sudo mount --bind /dev /wherever_you_have_mounted_your_system/dev
sudo chroot /wherever_you_have_mounted_your_system
mount -a

Now that you are in your system you can rebuild your initrd by reconfiguring Linux with the command

dpkg-reconfigure linux-image-<kernelversion>

Where <kernelversion> is the version of Linux you have installed that you wish to reconfigure. The default for Dapper is 2.6.15-23-386, but you can see all of the installed versions with (from within your system):

ls /lib/modules

If the reconfigure command was successful there should be some lines about rebuilding initrd, updating GRUB, etc. Now you can find your new initrd and matching kernel in /boot of your system (that is /wherever_you_mounted_your_system/boot in the Live CD's system) and they are called initrd.img-<kernelversion> and vmlinuz-<kernelversion>. You should exit from the chroot now by pressing ctrl-d. Now you need to copy these files, along with a configuration file for the bootloader GRUB, to the Live CD's home folder with the commands:

cp /wherever_your_system_is_mounted/boot/initrd.img-<kernelversion> ~
cp /wherever_your_system_is_mounted/boot/vmlinuz-<kernelversion> ~
cp /wherever_your_system_is_mounted/boot/grub/menu.lst ~

Now that you have a kernel which can boot your USB drive it is time to put it on a bootable CD. You will need to copy the "stage2_eltorito" file from inside your /usr/lib/grub folder to the Home folder of the live CD (the location varies slightly between 32bit and 64bit PCs). Now that you have the needed files we can make your CD. Make a folder in the Live CD's home folder called "bootcd", then inside there make another called "boot" and inside there one called "grub". Now copy the stage2_eltorito and menu.lst to this new "grub" folder and copy the vmlinuz and initrd files into the "boot" folder above it. Rename the vmlinuz-<version> file to simply "vmlinuz" and the initrd.img-<version> file to simply "initrd.img" (this means you won't have to change your configuration later to point to any new filenames) Edit the menu.lst file with the text editor and look at the bottom where the actual OS entries are. Delete what is there and make two new entries (replace /dev/sda1 with your root partition if this is not correct):

title          Ubuntu
root           (cd)
kernel         /boot/vmlinuz root=/dev/sda1 ro quiet splash
initrd         /boot/initrd.img
boot
title          Ubuntu Recovery Mode
root           (cd)
kernel         /boot/vmlinuz root=/dev/sda1 ro single
initrd         /boot/initrd.img
boot

By default the top entry will be booted. You can change some of the other options here if you want a hidden menu or a different timeout before the default entry is booted (never set this to zero or you will not be able to enter recovery mode). That is the entire contents of your bootable CD, so now we have to build it. Open a terminal (by default it will be in the home folder) and run (all on one line):

sudo mkisofs -R -b boot/grub/stage2_eltorito -no-emul-boot -boot-load-size 4 -boot-info-table -o UbuntuBootCDForUSB.iso bootcd

You should now have a CD image called UbuntuBootCDForUSB.iso which you can write to a recordable CD using many common CD writing programs such as the one built into the Nautilus file manager (Ubuntu's default file manager), GnomeBaker, k3b, Nero and many more. If you don't have your CDR/W drive accessible (because you are running a LiveCD in it) then you can copy the CD image to /wherever_you_have_mounted_your_system to keep it safely stored on a real drive. You should also copy over the "bootcd" folder. This can be done with:

sudo cp UbuntuBootCDForUSB.iso /wherever_you_have_mounted_your_system
sudo cp -r bootcd /wherever_you_have_mounted_your_system

They will now be accessible in your / directory (move them out of there once your system is set up as this is an important directory and should not be cluttered). You can save the iso file to a FAT formatted USB stick (obviously not one you have just set up with Linux) to transfer the CD image to Windows where it can be written by Nero. Now you just have to boot off the CD you have written whilst your USB drive is plugged in and (depending on how you set up GRUB) you will be sent into Ubuntu's boot procedure. (Notice the "Waiting for root filesystem" line? You did that!). If GRUB gives an error then you can edit the bootup lines by pressing "e" with the right entry highlighted in the menu.

Disadvantages Of This System

Whenever Ubuntu's kernel is updated you will not notice. This is because you will still be running the older kernel from your CD. You can fix this easily by replacing the "vmlinuz" and "initrd.img" files by the new ones in the "bootcd/boot" folder and making a new image by rerunning the above "sudo mkisofs" command (new kernels will automatically contain our modifications since we added the changes to the system's configuration files) After writing the new image to disc you can use it to boot into your new kernel (I would not recommend using a single CDRW disc and overwriting it everytime as you will be in trouble if it doesn't work. Instead I suggest using 2 CDRW discs and updating one. Once that updated disc has been tested and it works then you should update the second disc)

Booting the kernel from an internal hard drive

This section is only for those who have an internal drive that can be written to. It makes sense if you prefer to install Ubuntu to an external hard drive instead of the internal one, but have the possibility to customize the internal hard drive a little in order to boot ubuntu in your external drive which the BIOS does not recognize. Since the Ubuntu kernel is updated frequently, users need to update vmlinuz and initrd files in CD. There is a way to "Update kernel automatically" in CD but since it needs the kernel to be recompiled and some source code fixes, I'm not going to say more about that. Instead I'm going to tell you how to load initrd files and vmlinuz from disk. Most users will have an internal drive. After resizing /dev/sda1 in either GParted, create a new ext3 partition (anything more than 20MB should be enough). Note: You don't have to use an ext3 partition, but you need to use a partition type that grub can read, which is pretty much everything except NTFS. I.e. if your disk has a FAT32 partition already, you can copy vmlinuz and initrd there instead of making a new partition. Boot from the Ubuntu live cd (or boot from an older kernel). Mount the hard drive partition by using the terminal: (replace sda2 with your partition name)

sudo mkdir /vols
sudo mount /dev/sda2 /vols

and copy kernel and initrd to your hard drive partition:

cp /boot/vmlinuz-(your kernel version) /vols
cp /boot/initrd.img-(your kernel version) /vols

If you don't have grub installed on the hard drive, you'll need to run grub from a bootable CD like in the previous section. If you have grub on a CD-RW disk, add this to your menu.lst (replace the hd number with the one for your hard drive partition):

title  Ubuntu 7.04
root   (hd0,1)
kernel /vmlinuz-(your number) root=/dev/sdb1 ro quiet splash
initrd /initrd.img-(your version) 
boot

Or if you are using CD-R disks, please go to command line (c) when grub appears and type the same thing. Note: Replace sda, sdb with your situation.

See Also

• Building a bootable GRUB CD - This is GNU GRUB's official documentation which I used when writing this guide
• https://wiki.ubuntu.com/Booting

Comments

The instructions above assume an Ubuntu system in one partition. I had an extra partition for /boot and I ran into some troubles. In order to avoid this, build the CD from an Ubuntu system mounted generally in / .

• Thanks for the info, I added a line to bind /dev to the chroot and a mount -a command, which should take care of that. A /boot partition becomes basically redundant if you use this LiveCD method, so I don't think it would be a common situation. Personally I have only made /boot seperate when using LVM, but with a few extra modules this CD could boot directly to LVM, I may add that later (the CD image is tiny, so more features wouldn't hurt).

KBoot

For systems that won't boot from a USB device and/or don't even provide HDD/FDD emulation for USB devices in the BIOS, KBoot might be an option - it uses the Linux kernel itself as a boot loader. For example, you could burn a KBoot CD that could boot a kernel stored on a USB mass storage device. Maybe it could even be integrated in the live or alternate CDs? -- SteveDodd

• An installation guide for KBoot in Italian: Come installare Ubuntu su un hard disk esterno USB

Using UUID

In the section 'Booting the kernel from a bootable CD', I would suggest creating menu.lst with references to the UUID of the partition, not the device name. This tutorial has in mind a usb hard disk that boots always from the same computer, but you can also carry the disk with you, and use it to boot any computer. But then at every computer, your usb disk will be assigned a different device name. Just in case you are curious, the other modification you need to switch computers with the same external disk is simply delete /etc/X11/xorg.conf, so that it is created on the fly every time. X server configuration so much varies from one PC to another that you cannot use a single file for all computers. The rest goes about the same, and the CD created using this tutorial (with the UUID modification), works great.