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To boot or boot up a computer means to start it up, during which process, the operating system is loaded into memory. This is generally done by pressing a button on the computer itself, at which point the computer boots from its internal hard drive. The term boot disk, also bootable disk, is usually used to refer not to the built-in drive from which the computer normally boots but to a removable storage medium on which the operating system is stored and from which the computer can be started, if necessary. Since computers may be designed to boot up from their own hard drive, they can require a particular instruction given from the keyboard in order to start up from a boot disk.
A boot disk could be any of several different types of media, including a CD-ROM, a DVD-ROM, a flash drive, an external Firewire hard drive, or a floppy disk. In order for the boot disk to work, it must hold an operating system that is appropriate to the computer. You cannot, for example, boot a Windows computer with a boot disk holding a Macintosh operating system.
Many computers come with a boot disk. One type is LiveCD or LiveDVD. This refers to a version of the operating system that runs without needing to be installed. Additionally, there are specialized boot disks for sale.
A boot disk may have several functions, depending what is installed on it, if anything, besides the operating system. A bootable disk can be used when the computer cannot boot from its internal hard drive, or when that internal hard drive needs repair. In this case, booting from an external source allows the user to run a disk repair utility, which may be able to fix the internal hard drive and allow it to reboot. A bootable disk may also be employed in any attempts made at data recovery. Data recovery can be necessary if a computer’s hard disk fails, the computer is damaged, or for other reasons.
A bootable disk is also the mechanism for making changes to the operating system. A boot disk may be used to perform the initial installation of an operating system on a computer. It may also be used to reinstall the operating system, which might be necessary if, for example, one needed to securely erase the data on the system before selling it or in ridding the computer of a virus. It can also be used to replace an operating system, that is, upgrade it to a newer version or downgrade it to a previous version.
Keep in mind that a lot of people forget to reconfigure their BIOS so that something other than a hard drive can be used as a bootable disc. If you are trying to install a new operating system from a flash drive or trying to run a recovery disc, that might not be possible unless you tell your system to boot from one of those at startup. That is a real problem if your hard drive is dead and you can't get your computer to work because the system can't boot from it.
To alter your BIOS, look for the prompt at startup that tells you which key to press to enter into the BIOS setup. From there, you can set your boot discs.
If you get lost in that process, consult your owner's manual to figure out exactly how to enter the BIOS setup menu.
The live disc is one of the things that makes it possible to shop for various versions of the Linux operating system. There are plenty of ways to make a USB drive a bootable disc from which a Linux distro can be run and tested before it is installed.
That's a far cry from the old days when one had to burn an operating system to a CD or DVD ROM before trying it. That actually cost some money for the media, whereas most people have a USB lying around that can be used, format and then used again.