A bootable disk is a storage medium such as a CD, DVD, USB flash drive, or disk partition that contains operating system installation files and utilities that allow a computer to startup (boot) and install an operating system. The key contents of a typical bootable disk include:
The bootloader is code that initializes the boot process and loads the operating system kernel. For PCs, the standard bootloader is the Grand Unified Bootloader (GRUB). When you start a computer from a bootable disk, the BIOS looks for the Master Boot Record (MBR) and executes the bootloader code stored there, which contains instructions for loading the operating system.
The kernel is the core software component of the operating system that bridges the gap between a computer’s hardware and software. It manages system resources, sets up memory addressing, handles multitasking, and more. A bootable disk will contain a minimal kernel, just enough to load the rest of the operating system from disk or over the network.
Initial RAM Disk
Many bootable disks use an initial RAM disk (initrd) which is a temporary, memory-resident file system loaded during booting. It contains key drivers and modules needed to detect hardware and mount the permanent root file system. The initrd allows the system to boot even if certain storage drivers are unavailable.
Operating System Files
A boot disk contains the minimum critical operating system files needed to fully load and run the OS. For Linux, this includes system initialization scripts like rc.sysinit, basic filesystem and shell binaries, and the core system libraries and modules. For Windows, this includes the Windows PE (Preinstallation Environment) files.
In order to support various system recovery, maintenance, and troubleshooting scenarios, boot disks typically include a range of diagnostic and repair tools. Examples include memory testers, disk partition editors, filesystem checkers, system monitors, and network utilities.
To allow the user to select between multiple boot options, boot managers like GRUB, Syslinux, or Bootmgr are included on the bootable disk. They detect and list available operating systems or environments, and pass control to the selected one.
To enable user interaction during the boot process, boot disks usually include a minimal command line or graphical interface. This allows running utilities, editing configurations, and interpreting output before the full OS loads.
Support for critical hardware like storage controllers, network adapters, and graphics chips is provided by a basic set of drivers on the boot disk. This ensures the OS can be installed and operated on that particular system.
Tools for creating, checking, and repairing different filesystems like FAT, NTFS, HFS+, ext4, XFS, and ZFS are typically part of a boot disk. This allows managing partitions and data recovery across many disk formats.
For network booting and post-boot network access, boot disks include networking binaries and daemons like DHCP clients, SSH, and network file transfer programs. This allows retrieval of OS images and remote management.
Some boot disks are designed specifically for security analysis and data recovery and include password resetters, disk editors, network sniffers, and forensics tools to examine systems and data.
Boot disks can be customized with additional binaries and scripts for niche tasks like cloning disks, automating installs, gathering system inventory data, or running benchmarks.
The full OS installer, or network image boot files, can be included to automate operating system deployment across many systems.
How are Bootable Disks Created?
There are a few common ways bootable disks are built:
- Optical disk images – ISO files containing bootable contents are written to CD/DVD media.
- Partition images – Partition images with bootloaders are written to USB drives or other disks.
- Hybrid USB drives – Contain both bootloader and partition images for booting and data.
- flashing firmware on read-only media to make it bootable, like ROM chips.
Dedicated utilities like mkisofs, dd, and Rufus handle creating bootable media from disk images containing bootable structures and data.
Types of Bootable Disks
Some common bootable disk types and their uses include:
|Linux Live CD/USB
|Boots a lightweight Linux distribution running from RAM
|System recovery, rescue, and desktop usage
|Minimal Windows Preinstallation Environment
|Windows deployment and diagnostics
|Hiren’s Boot CD
|Collection of various boot utilities and tools
|Disk management, diagnostics, and data recovery
|Network boot images
|Downloads OS image over network
|Network installation of operating systems
|Forensics boot disk
|Boots a digital forensics focused toolkit
|Investigation of computer systems
|Custom boot disk
|User-defined tools and operating system
|Specific system maintenance, recovery, or install tasks
Key Bootable Disk Tools
Some examples of commonly included tools are:
- fdisk – Partition table editor
- fsck – Filesystem checker and repair tool
- dd – Disk cloning and copying utility
- chkdsk – Disk repair utility in Windows
- gparted – Gnome Partition Editor with GUI
- Photorec – File recovery utility
- DBAN – Disk wiping utility
- Clonezilla – Disk cloning and imaging tool
- SystemRescueCD – Boot utility with a host of tools
- Ophcrack – Password cracking/recovery tool
Creating a Bootable USB Drive
On Windows, you can create a bootable USB drive with these steps:
- Download the ISO file for your desired bootable disk or OS installer.
- Insert your target USB drive into your computer.
- Open Rufus app and select your ISO file and USB drive.
- Click start to format and create bootable USB.
- Boot computer from the flash drive to launch boot disk.
On Linux or macOS, the command line dd tool can be used:
dd if=file.iso of=/dev/sdX bs=4M status=progress
Where sdX is your USB drive. This copies the ISO byte-for-byte directly to the drive.
Booting from a Disk
To boot from your new bootable disk on a PC:
- Insert the prepared bootable CD/DVD into the drive, or USB into a port.
- Power on the computer and enter the BIOS setup (by pressing F2, F10, Delete etc.).
- Change boot order to position your boot disk drive as the first boot device.
- Save changes and exit BIOS to reboot from the bootable disk.
On Macs, hold the Option key after restarting to select the bootable disk from the boot manager.
Uses for Bootable Disks
Some common uses for bootable disks include:
Operating System Installation
Boot disks allow starting the installation process for operating system deployment across many computers. The OS media contains the installer and boot components.
When a computer won’t boot normally, boot disks provide access to utilities for analyzing, repairing, and restoring systems to a working state.
Resetting forgotten user account or system passwords is made possible via password reset tools on specialized boot disks.
Boot disks provide a means to access and recover data from corrupt or failing drives that are unreadable by normal booting.
Investigators use forensics boot disks with specialized tools to examine systems for potential evidence or analyze cyber incidents.
Boot disks with networking utilities are helpful for troubleshooting connectivity issues or networking services.
System diagnostics and hardware tests can be performed from a minimal boot disk environment for troubleshooting.
Reformatting, overwriting, or securely erasing disk drives is made possible using wiping tools on boot disks.
Advantages of Bootable Disks
Key advantages to using boot disks include:
- Independence from installed OS – disks provide a separate environment.
- Access to data – boot files when drives are unreadable otherwise.
- Recovery ability – fix boot issues and restore systems.
- Secure wiping – completely erase data from disks.
- Hardware testing – diagnostics not limited by OS constraints.
- Forensics – investigate systems for hidden issues or evidence.
- Network capability – troubleshoot network services and configuration.
- Mobility – carry useful troubleshooting tools conveniently.
Bootable disks are an indispensable tool for technicians, system administrators, and advanced computer users. They provide a self-contained operating system environment loaded with utilities for many usage scenarios like deployment, recovery, diagnostics, forensics, networking, data access, hardware testing, and more. Typical contents include bootloaders, kernels, boot managers, command line interfaces, hardware drivers, disk utilities, and custom tools. Reasons to use boot disks include the ability to boot apart from installed operating systems, access drives and data, securely erase data, run hardware diagnostics, probe network issues, recover forgotten passwords, perform forensics, and troubleshoot a wide range of problems.