A bootable device is any type of storage device that contains an operating system and can be used to start up or “boot” a computer. When a computer boots up, it looks for a bootable device to load the operating system from. Some common examples of bootable devices include internal hard drives, external hard drives, USB flash drives, CDs, and DVDs.
What Happens During Bootup?
When you first turn on your computer, it goes through a boot up sequence to load the operating system. Here are the basic steps:
- The BIOS (Basic Input/Output System) runs a Power On Self Test (POST) to check that the basic hardware is working.
- The BIOS looks for bootable devices that are connected to the computer and checks their boot order priority. This boot order is usually configurable in the BIOS settings.
- If the BIOS finds a bootable device, it loads and executes the Master Boot Record (MBR) from the boot sector of that device.
- The MBR contains information on how to load the operating system kernel and starts the bootloader.
- The bootloader loads the operating system kernel into the computer’s RAM and transfers execution to the kernel.
- The kernel initializes the rest of the operating system, loads drivers, and starts all background services to get the computer into an operational state with a usable desktop environment.
This whole boot up process depends on having a bootable device connected to the computer that contains the operating system files. Without a bootable device, the computer would not know how to load the operating system and would not be able to start up properly.
What Makes a Device Bootable?
For a storage device to be bootable, it must have an operating system installed on it and be partitioned correctly. Here are the key requirements:
- Operating system files – The device needs to contain the critical operating system files like the kernel, bootloader, and device drivers.
- Master Boot Record – A boot sector at the beginning of the drive has to contain bootloader code and information on the disk partitions.
- Bootloader – Bootloader code needs to be installed that can load the operating system kernel.
- Partition table – The drive requires a partition table defining the partitions on the drive, including a bootable partition with a file system.
With these elements set up properly, the BIOS can detect the device as bootable media and start the bootup sequence from it.
Common Bootable Devices
There are many different types of devices that can be made bootable. Here are some of the most common bootable devices:
Internal Hard Drives
The primary hard drive installed inside a computer is the most common bootable device. Windows, Linux, and other operating systems are almost always installed to an internal hard drive partitioned and configured to be bootable.
External Hard Drives
External USB hard drives can also be made bootable by installing an operating system to a partition on the drive. For example, you can install a portable version of Windows or Linux on an external drive and boot from it on any computer.
USB Flash Drives
USB flash drives and SSDs are a very popular type of bootable device, especially for portable operating system installations. For example, Linux distributions like Ubuntu have USB installer versions that can make a flash drive bootable.
Optical Discs
CDs, DVDs, and Blu-ray discs can be made bootable by writing an operating system installation image to them. Bootable optical discs are often used for OS installation, recovery, diagnostics, and more.
Network Booting
On corporate networks, large server computers can act network boot servers. This allows client computers to boot over the network from an operating system image on the server rather than locally.
Making a USB Drive Bootable
One very useful application of bootable devices is to create a bootable USB drive. This allows you to carry a portable operating system with you that can boot on any computer. Here is an overview of how to make a USB drive bootable:
- Download the ISO file for the operating system you want to make bootable. This could be something like a Windows or Linux installer image.
- Use a utility like Rufus on Windows or Startup Disk Creator on Linux to write the ISO to your USB drive.
- The utility will set up the USB partition, file system, boot files, and copy the ISO contents.
- Configure your computer’s BIOS to boot to the USB drive by selecting it in the boot order settings.
- Boot your computer from the USB drive. It will load the operating system files from the drive.
Once you have a bootable USB drive, you can boot and run that operating system installation on any computer that supports booting from USB. This provides a very flexible way to have a portable operating system toolbox.
Advantages of Bootable Devices
Using bootable devices provides a number of advantages:
- Portability – Bootable USB drives allow you to carry an operating system with you and use it on any computer.
- Isolation – Booting from a separate device allows you to keep the host computer’s existing OS intact.
- Recovery – Booting from external media is useful for recovering data from corrupted systems.
- Installations – Installers for operating systems are often made as bootable optical discs or USB drives.
- Diagnostics – Booting specialized operating systems from external devices allows you to run diagnostics.
- Security – Some bootable media like Tails Linux provide enhanced security and privacy.
Types of Bootable Operating Systems
There are many different operating systems that are designed to boot from external devices. Here are some examples:
Windows PE
Windows Preinstallation Environment (PE) is a minimal Windows system commonly used for installation, deployment, diagnostics, and recovery. It boots from USB or DVD.
Windows To Go
Windows To Go is a full featured Windows installation that can boot and run from a large USB drive. It provides a portable Windows environment.
Linux Distributions
Most Linux distributions like Ubuntu, Mint, and Fedora allow you to create a “Live USB” that boots up a running Linux system from a flash drive.
Tails Linux
Tails is a privacy-focused Linux distro designed to boot from USB and leave no trace on the host computer when shut down.
GParted
GParted is a small Linux distro focused specifically on disk partitioning. It is useful for preparing drives before installations.
Hiren’s BootCD
Hiren’s BootCD contains a variety of diagnostics and recovery tools bundled together in a bootable mini operating system.
These types of dedicated bootable operating systems allow you to perform all sorts of specialized tasks from a single external device.
How to Boot from a CD/DVD
Booting from optical discs like CDs and DVDs used to be very common. Here is an overview of the process:
- Check that your computer’s BIOS supports booting from the optical drive.
- Insert the bootable CD/DVD into the drive and restart the computer.
- Access the BIOS settings (typically by a key like F12, F2, or Delete) and select the optical drive as the first boot device.
- Save changes and exit the BIOS. The computer should boot from the disc.
- When done, eject the disc and reset the boot order back to the hard drive.
Bootable optical discs are not as widely used now compared to USB drives. However, they are still useful for some installation and recovery scenarios.
Network Booting
Enterprise environments with many computers often use network booting to load operating systems over the network rather than local drives. This uses a Pre-Boot Execution Environment (PXE) server.
The basic process for network booting is:
- The PXE server provides a DHCP service assigning IP addresses to clients.
- Clients broadcast a DHCP request to get an IP address and PXE config info.
- The server assigns an IP and sends a boot loader to the client.
- The boot loader triggers the client to request the OS image from the PXE server.
- The server provides the OS image which boots on the client machine.
Network booting allows for consistent standardized OS installations across many machines. It is very common in server racks and computer labs.
Booting from Internal Hard Drives
Booting from the primary internal hard drive is the default method used on most computers. The hard drive is partitioned with a boot sector and bootloader installed. Here is how booting works internally:
- The BIOS checks the first sector of the hard drive called the Master Boot Record (MBR).
- The MBR contains bootloader code and info on disk partitions.
- The bootloader located active primary partition with OS installed.
- Bootloader loads OS kernel files from the active partition into RAM.
- Kernel initializes rest of OS, services, drivers, etc. to take over.
This boot process is initiated automatically on modern computers with internal hard drives containing operating systems.
Dual-Booting Multiple OS
It is possible to setup a system to dual-boot between multiple operating systems on the same computer. This involves partitioning the hard drive with separate partitions for each OS. Here is how dual-booting works at a high level:
- Partition hard drive into separate partitions for each OS install.
- Install each OS to its own partition.
- Install a boot manager like GRUB that lets you select which OS to load at boot time.
- At boot, the boot manager allows you to pick which partition to boot from.
- Bootloader then loads that selected OS partition.
With this setup, you can reboot and select which operating system you want to run. This allows you to have multiple operating systems on the same computer.
Troubleshooting Boot Issues
There are a number of common problems that can prevent a computer from booting properly:
- Invalid boot device – BIOS cannot find a bootable device. Check connections and boot order settings.
- Bootloader corrupted – Rebuild Master Boot Record and reinstall bootloader.
- Missing operating system – Install OS to bootable partition or replace damaged hard drive.
- Hard drive failure – Replace failed hard drive if it cannot boot.
- Hardware failure – Faulty RAM, CPU, motherboard can cause boot errors.
- Bootloader misconfiguration – Fix bootloader to point to correct OS partition.
Troubleshooting boot problems requires patience and technical knowledge to pinpoint the exact cause. Start with simple solutions first before investigating deeper hardware and configuration issues.
Conclusion
Having a bootable device is essential for starting up any computer and loading an operating system. Internal hard drives, external USB drives, optical discs, and network PXE boot can all serve as valid bootable devices if properly configured with the necessary boot files and operating system. Understanding what makes a device bootable and how the boot process works helps diagnose and fix boot problems that may occur.