There are many good reasons for booting your computer from a USB drive. For instance, if you want to try out another operating system without disturbing the main hard drive on your computer, then installing the new OS on an external drive is a great way to run the system through its paces and decide whether you like it.
This approach will offer you the flexibility of being able to run both operating systems on your computer. However, before you can boot the new operating system from the USB drive, you will need to change the boot order for your computer in the system BIOS.
Connect the external USB drive or pen drive to the computer before you power it on. Press the "Delete" key as the computer performs the "Power On Self Test" (POST) to enter the BIOS area of the computer firmware. The POST performs a diagnostic of components on the motherboard and does a cursory check of the hard drive, memory and processor. The POST only lasts a few seconds; therefore, press the "Delete" key quickly. Enter the administrative BIOS password if prompted to do so. Once you enter the correct password, you will see the main BIOS screen. Use the right-arrow key to navigate to the "BOOT" menu option at the top of the screen, and press the "Enter" key. Press the down-arrow key until the cursor is on the "Boot Device Priority" label. Hit "Enter," and press the down-arrow key again to highlight the "1st Boot Device" option. Press the "+" or "-" keys to scroll through the available boot options until the "USB Drive" or "USB-HDD0" label is the highlighted option. Press the "F10" key to save changes and exit the BIOS setup program. When the computer restarts, wait for the "press any key to boot from external drive" prompt to display, and then press any key on the keyboard to boot from the USB drive. Award or Phoenix BIOS Connect the USB drive to the computer. Wait for the POST to display on the screen, and press the "Delete" key to enter the BIOS. Enter the BIOS password if prompted to do so. Tap the right-arrow key until the cursor is over the "BOOT" menu option. Press the "Enter" key. Press the down-arrow key until the highlighted cursor is on the "Removable Devices" option. Press the "+" key until the "Removable Devices" label is on the top of the boot device list. Press the "F10" key to save the changes for the boot order in the BIOS and to exit the utility. After the computer restarts, press any key on the keyboard to boot from the USB drive when prompted to do so.
Bootable USB guide, here we assume that you are using either Vista or Windows 7 to create a bootable USB.
1. Insert your USB (4GB+ preferable) stick to the system and backup all the data from the USB as we are going to format the USB to make it as bootable.
2. Open elevated Command Prompt. To do this, type in CMD in Start menu search field and hit Ctrl + Shift + Enter. Alternatively, navigate to Start > All programs >Accessories > right click on Command Prompt and select run as administrator.
3. When the Command Prompt opens, enter the following command:
DISKPART and hit enter.
LIST DISK and hit enter.
Once you enter the LIST DISK command, it will show the disk number of your USB drive. In the below image my USB drive disk no is Disk 1.
4. In this step you need to enter all the below commands one by one and hit enter. As these commands are self explanatory, you can easily guess what these commands do.
SELECT DISK 1 (Replace DISK 1 with your disk number)
CREATE PARTITION PRIMARY
SELECT PARTITION 1
(Format process may take few seconds)
Don't close the command prompt as we need to execute one more command at the next step. Just minimize it.
5. Insert your Windows DVD in the optical drive and note down the drive letter of the optical drive and USB media. Here I use "D" as my optical (DVD) drive letter and "H" as my USB drive letter.
6. Go back to command prompt and execute the following commands:
D:CD BOOT and hit enter. Where "D" is your DVD drive letter.
CD BOOT and hit enter to see the below message.
(Where "H" is your USB drive letter)
7. Copy Windows DVD contents to USB.
You are done with your bootable USB. You can now use this bootable USB as bootable DVD on any computer that comes with USB boot feature (most of the current motherboards support this feature).
Note that this bootable USB guide will not work if you are trying to make a bootable USB on XP computer. http://www.howtogeek.com/howto/16822/boot-from-a-usb-drive-even-if-your-bios-wont-let-you/
boot from a usb drive