Boot media refers to the medium or device that contains the bootloader and operating system files that allow a computer to start up or “boot”. When a computer is powered on, the basic input/output system (BIOS) or unified extensible firmware interface (UEFI) firmware initializes the hardware and looks for a bootable device to load the operating system from. Some common examples of boot media include:
Hard Disk Drives (HDDs)
Most desktop and laptop computers use internal hard disk drives as the primary boot medium. An HDD stores the operating system files, bootloader, and other data needed to fully load the operating system. At startup, the BIOS checks the HDD for bootable partitions and executes the bootloader to initialize the operating system.
Solid State Drives (SSDs)
SSDs are increasingly being used as boot media, especially in newer computers. SSDs provide faster boot times than traditional HDDs due to faster read/write speeds and access times. The boot process works the same way as with HDDs – the SSD contains the bootloader and OS files which are loaded at startup.
Optical Discs
CDs, DVDs, and Blu-ray discs can be used as boot media. Optical drives can be configured as a first boot device in the BIOS, allowing you to boot the computer from an OS installation disc or a bootable disc with a live OS environment.
USB Flash Drives
USB flash drives are a very common boot medium because they are inexpensive, portable, and removable. Bootable USB drives can be created with an OS installer or disk image file, allowing you to boot into an OS on any computer from the flash drive.
Network Booting
Some operating systems support booting over the network via PXE (Preboot Execution Environment) or other network boot protocols. This allows a client computer to remotely boot and load the OS from a server on the local network.
BIOS/UEFI Boot Options
The BIOS or UEFI firmware provides a boot order or priority that determines which devices are checked for bootable media. Typical options include:
- Hard drive
- Removable drives like USB and optical drives
- Network interface for PXE/network booting
The boot order can be changed through the BIOS/UEFI setup utility to select the desired boot device.
Bootloaders
A bootloader is a small program that loads the operating system kernel and essential drivers needed to get the OS running. The bootloader resides in the first sector of the boot disk and runs automatically when the computer starts.
Some common bootloaders include:
- GRUB (GRand Unified Bootloader) – used by Linux distributions
- NTLDR (New Technology Loader) – earlier Windows versions
- BOOTMGR – newer Windows versions
The bootloader has information about available operating systems and configurations and allows the user to select which one to boot into.
Boot Partitions
A boot partition is a partition on the boot disk specifically designated to hold files critical for system bootup. This partition is protected from changes that could render the system unbootable.
A boot partition typically includes:
- The bootloader
- The kernel of the OS
- Other core files needed before the actual OS loads
Having a separate boot partition helps prevent OS issues or changes from making the computer unbootable. The boot files are isolated from the rest of the system.
Master Boot Record (MBR)
The master boot record (MBR) is a special type of boot sector located in the first sector of a drive. The MBR contains information about disk partitions and has code to load a bootloader from the boot partition.
Key facts about the MBR:
- Resides in first sector of disk (512 bytes in size)
- Contains a disk signature unique to the drive
- Includes a partition table detailing partitions on disk
- Has executable boot code to load the bootloader program
The MBR is used as first stage of traditional BIOS booting. The MBR boot code executes, reads the partition table, identifies the boot partition, and loads and executes the bootloader program.
GUID Partition Table (GPT)
GUID Partition Table (GPT) is the newer standard partition style used instead of MBR on UEFI systems. GPT provides a more flexible and reliable approach to partitioning and booting.
Advantages of GPT over MBR:
- Supports disks over 2TB in size
- Provides redundancy and error checking with primary and backup tables
- Uses globally unique identifiers (GUIDs) for partitions
- Supports unlimited partitions (128 max in MBR)
GPT is part of the overall UEFI boot process. The system reads the GPT partition information to identify bootable partitions and loads files stored on them.
Boot Menu
The boot menu displays when the boot order has multiple bootable devices or operating systems. This menu allows you to selectively choose which device or OS to boot from.
The boot menu can typically be accessed in different ways:
- By pressing a function key like F12 or F10 as the computer starts up, before the primary OS loads.
- From the BIOS/UEFI settings screen by selecting the “Boot” options.
- By configuring the timeout delay for the menu within the BIOS/UEFI firmware settings.
This provides a convenient way to select between multiple boot partitions or devices when starting up your system.
Bootable Devices List
Here is a table summarizing some common bootable devices and media types:
| Device | Description |
|---|---|
| Hard disk drive (HDD) | Internal HDDs are the primary boot device in most computers. |
| Solid state drive (SSD) | SSDs are increasingly used as the primary boot drive. |
| USB flash drive | Inexpensive and portable boot medium. |
| Optical disc | CD, DVD, and Blu-ray discs can be bootable. |
| External HDD | External portable hard drives can be booted from if marked as bootable. |
| Network (PXE) | Client computers can network boot from a PXE server. |
Creating Bootable Media
There are various methods to create bootable media depending on the device and operating system.
Hard Drives
Internal hard drives are made bootable by installing an operating system to a partition on the drive. The OS installer copies the necessary boot files including the bootloader to the drive as part of the installation process.
USB Drives
Dedicated tools allow you to write an OS image (.iso) to a USB drive and make it bootable. For example:
- Rufus – Utility for Windows
- Disk Utility – Tool in macOS
- dd – Command line tool on Linux and macOS
- Etcher – Cross-platform graphical tool
Optical Media
Programs like CD/DVD burning software on Windows, or utilities like dd on Linux, allow you to write disk image files to create bootable optical discs.
Boot Errors
Some common boot errors include:
- Invalid partition table – The partition table describing disk partitions may be corrupt or missing.
- Missing operating system – No valid bootable operating system is detected.
- BOOTMGR missing – The Windows boot manager files are corrupted or missing.
- NTLDR missing – Windows XP boot loader files are missing or damaged.
- No boot device found – BIOS/UEFI can’t find a bootable device.
- Boot sector virus – The master boot record (MBR) or boot sector is infected by a virus.
These errors typically occur after changing boot order, partition structure, boot files, or OS installations. Boot errors can often be fixed by repair tools or reinstalling operating systems.
Conclusion
In summary, boot media provides the storage location containing the operating system files and bootloader code required for a computer to start up. The BIOS/UEFI firmware initializes the hardware on power up, checks potential boot devices based on a boot order, locates bootable media, and executes the bootloader program to load the OS kernel and system files from storage into memory.
Hard drives are the most common boot media, however many different devices and media can also be used to store and deliver the files needed for system boot. Understanding the boot process and the various boot methods available is helpful for troubleshooting issues and maximizing performance.
