Booting from a USB Thumb Drive

Since USB thumb drives are getting enormous capacities it makes sense to get the best use of them by installing an operating system on them. With a bootable thumb drive you can plug it into any computer and boot to a different operating system. It doesn't even matter what's already installed on the other computer, the thumb drive will over-ride it since it was configured in the BIOS. This has to be the most portable, full size, razzle dazzle computer possible (for now, at least).
Most computers have the option in the BIOS to enable booting from USB drives so check it out. Access the BIOS during boot (hit Del or the F10 key usually). In the BIOS menu select boot devices and enable whichever USB device best describes your USB thumb drive. This option can be can be the first in the boot order list of devices since it will only matter when the thumb drive is inserted. The BIOS configuration takes precedence over anything on disk since it gets loaded long before any installed operating system. That's why the USB thumb drive can ignore the OS on disk.


While people use the terms, BIOS and CMOS interchangeably, they're two different things. CMOS (Complementary Metal Oxide Silicon) describes the technology of the IC used to store the startup information for a computer. It's where intructions are stored for booting the machine. The information stored on the CMOS chip is kept fresh by a battery. If that battery fails or is removed all the stuff stored in the CMOS chip is lost.

The BIOS (Basic Input/Output System) is the information that gets stored in the CMOS chip. When the computer starts to initialize it queries the BIOS for installed devices like hard disks, CD drives, the amount of memory and all the other odds and ends about the physical devices on the computer. You can edit the BIOS if you need to change something though that's rarely necessary. You will mostly access the BIOS to confirm that all the devices were correctly recognized.

What all this means means is that you edit the BIOS settings, not the CMOS. This distinction probably doesn't matter but it's handy to understand what you're actually doing before you talk about it.

Once the USB's operating system has booted you can use everything that OS has to offer as if it had been installed to the computer's hard disk. This is really handy if the host computer (the one you plugged the thumb drive into) is hosed by a virus or misconfiguration or won't boot. You can have a bunch of cool utilities for repairing the host system. If the thumb is a Linux install, you can mount the host drives and work on them directly from Linux.

Use the Linux fdisk to see what partitions live on the host disk and their file system type. Create a mount point directory (/media/windows, for instance) and mount it. Let's say we learned that Windows is on /dev/hda3, do this:

mount -t NTFS /dev/hda3 /media/windows
You should be able to see the windows stuff by doing a cd /media/windows and then do a ls -l. Now you can fix whatever's broken. This is about the same as using a "live" CD like Knoppix except a thumb drive is a whole lot faster and a lot easier to carry around. If you've installed a bunch of Windows utilities on your thumb drive, you can use them to manage the Windows install, recover passwords, cleanup temp files or whatever else seems useful.

A Thumb drive is also a good place to keep odds and ends that you want to add to other computers. You don't have to make the thumb drive bootable if all you want to do is have all your favorite utilities handy. Maybe you want to store an entire website on the thumb drive as a backup and work on the files on another computer. If you've installed the Apache web server and all your web pages on the thumb drive, you can turn any other computer into a web server and full blown web site by just inserting the thumb drive.

I have had problems getting some thumb drives to boot correctly so do some research before you buy one for booting with. I'm not sure that the Microsoft compatible software, Readyboost, makes any difference but be aware that it's installed on most thumb drives. In most cases the Linux install will recognize the thumb drive as a boot target and offer it as an option for installation. If so the Linux install will work fine. If there's some issue check the BIOS settings to make sure you're using the right USB device and that you can boot from it.

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