转自：https://help.ubuntu.com/community/Installation/FromUSBStick

步骤：

原文摘抄如下：

Create Bootable USB Manually

Here is a way to create a bootable USB flash drive manually. The advantage is you don't have to empty your USB flash drive as long as you have enough space for the files. It also allows multi boot, so you can, for example, have your USB flash drive holding both 32bit and 64bit versions of Ubuntu and other Boot CD tools at the same time.

In order to do so you need : 

• Bootable USB flash drive ( NTFS / FAT32 / FAT 16 ), Disk with MBR, you can do this with gparted in linux or HP format in Win, fdisk ....etc
• Ubuntu ISO of your choice
• Tools to extract files from the ISO ( ISO Mount, WinZIP, WinRAR, 7Zip, etc....)

• Grub4Dos 

  1. Grub4DOS' Tutorial

  2. Grub4DOS 0.4.4 for WIN9x/DOS/Linux(Download Here)

  3. Grub4DOS GUI for WINXP/Vista/7

First you need to install grub onto the USB's MBR. Follow the link to the Grub4DOS Wiki: Simple example:

./bootlace /dev/sdx

For more details, visit: http://sourceforge.net/projects/grub4dos/

Next, you open up the ISO file and extract the files in casper directory. The size (and possibly manifest) are needed if you want to install from the USB flash drive otherwise the install will fail.

total 701060
-rwxr-xr-x 1 adrian adrian     38784 2010-07-27 16:15 filesystem.manifest
-rwxr-xr-x 1 adrian adrian        10 2010-07-27 16:15 filesystem.size
-rwxrwxrwx 2 adrian adrian 704487424 2010-04-29 05:38 filesystem.squashfs*
-rwxrwxrwx 1 adrian adrian   9365872 2010-04-29 05:34 initrd.lz*
-rwxrwxrwx 1 adrian adrian   4029792 2010-04-16 06:01 vmlinuz*
/media/Fujitsu 60GB USB/bootimg/ubuntu.10.4.x86$

Copy grldr to the root of the drive.

drwx------ 1 adrian adrian   4096 2010-05-05 16:49 bootimg/
-rwxrwxrwx 1 adrian adrian 220049 2009-09-24 17:30 grldr*
-rwxrwxrwx 1 adrian adrian   2760 2010-05-04 23:08 menu.lst*
/media/Fujitsu 60GB USB$

Now create menu.lst at the root of USB. Example of menu.lst:

default /default
##################### 
title Ubuntu 10.04 LiveCD
find --set-root /bootimg/ubuntu.10.4.x86/initrd.lz
kernel /bootimg/ubuntu.10.4.x86/vmlinuz boot=casper live-media-path=/bootimg/ubuntu.10.4.x86/ ignore_uuid
initrd /bootimg/ubuntu.10.4.x86/initrd.lz
boot

Make sure the path match where the files you copy to.

Now, reboot.

Simpler way using the ISO file

Another way is just using the ISO file as it is on an already bootable USB flash drive that uses Grub (to create one, see above).

• copy the iso file onto the USB flash drive in the root (top) directory
• add these lines to the menu.lst file:

title Ubuntu 11.04 i386
find --set-root /ubuntu-11.04-desktop-i386.iso
map /ubuntu-11.04-desktop-i386.iso (0xff)
map --hook
root (0xff)
kernel /casper/vmlinuz file=/cdrom/preseed/ubuntu.seed boot=casper iso-scan/filename=/ubuntu-11.04-desktop-i386.iso splash quiet --
initrd /casper/initrd.lz

Note: this might not work with all versions of grub. It was tested with GRUB4DOS 0.4.4 2008-11-11