Boot disks and live CDs

A boot disk is removable media — historically a floppy disk, later a CD, DVD or USB stick — that a computer can start from instead of its internal hard drive. When the firmware (BIOS or UEFI) is set to check removable media first, it loads a small operating environment from the disk. This is invaluable when the installed operating system will not start, because it provides a working system from which to diagnose and repair the machine.

A live CD (or live USB) takes this further by running a complete operating system entirely from the removable media and system memory, without installing anything or changing the contents of the hard drive. This lets a technician boot into a clean, known-good environment to back up files, scan for malware, edit partitions, or test hardware on a computer whose normal system is broken or compromised. Linux distributions popularised the live-CD approach, and lightweight tools such as boot-repair and antivirus rescue discs follow the same idea.

Bootable media are commonly built for specific repair tasks: emergency recovery discs that restore a damaged installation, partitioning and imaging tools that clone or resize drives, password-reset utilities, and registry-repair environments for Windows. Because they run independently of the installed system, they can fix problems that cannot be addressed while that system is running. Many such tools are freely available, and creating bootable USB media has largely replaced the older floppy and optical boot disks.

Frequently asked questions

What is a live CD?

It is media that runs a full operating system from the disc and memory without installing anything, leaving the hard drive untouched.

Why use a boot disk?

It lets a computer start from removable media when the installed system will not boot, providing a working environment for repair, recovery or diagnostics.

Can a boot disk be on a USB stick?

Yes. Modern firmware can boot from USB, and bootable USB drives have largely replaced older floppy and CD boot disks.