What Is a Master Boot Record (MBR)?

Definition of MBR & how to fix missing or corrupt MBRs

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A master boot record (often shortened as MBR) is a kind of boot sector stored on a hard disk drive or other storage device that contains the necessary computer code to start the boot process.

The MBR is created when a hard drive is partitioned, but it's not located within a partition. This means non-partitioned storage mediums, like floppy disks, don't contain a master boot record.

The master boot record is located on the first sector of a disk. The specific address on the disk is Cylinder: 0, Head: 0, Sector: 1.

The master boot record is commonly abbreviated as MBR. You might also see it called the master boot sectorsector zeromaster boot block, or master partition boot sector.

What Does the Master Boot Record Do?

A master boot record consists of three major pieces: the master partition table, the disk signature, and the master boot code.

Here's a simplified version of the role the master boot record plays when a computer is first starting up:

  1. BIOS first looks for a target device to boot from that contains a master boot record.

  2. Once found, the MBR's boot code uses the volume boot code of that specific partition to identify where the system partition is.

  3. That particular partition's boot sector is then used to start the operating system.

As you can see, the master boot record plays a very important job in the startup process. Without this particular section of instructions always available, the computer would have no idea how to start Windows or whatever operating system you're running.

How to Fix Master Boot Record (MBR) Problems

Issues with the master boot record can happen for various reasons — maybe a hijacking by an MBR virus, or maybe corruption thanks to a physically damaged hard drive. The master boot record might be damaged in a small way or even removed entirely.

A "No boot device" error usually indicates a master boot record problem, but the message could be different depending on your computer maker or motherboard's BIOS manufacturer.

An MBR "fix" needs to be performed outside of Windows (before it starts) because, of course, Windows can't start.

Some computers will attempt to boot from a floppy before a hard drive, in which case any sort of malicious code that's on that floppy will then be loaded into memory. This type of code can replace the normal code in the MBR and prevent the operating system from starting.

If you suspect that a virus could be to blame for a corrupt master boot record, we recommend using a free bootable antivirus program to scan for viruses before the operating system starts up. These are like regular antivirus programs but work even when the operating system doesn't.

MBR and GPT: What's the Difference?

When we talk about MBR and GPT (GUID Partition Table), we're talking about two different methods of storing partition information. You'll see an option to select one or the other when you're partitioning a hard drive or when you're using a disk partitioning tool.

GPT is replacing MBR simply because it has less limitations than MBR. For example, the maximum partition size of an MBR disk that's formatted with a 512-byte unit allocation size is a measly 2 TB compared to the 9.3 ZB (over 9 billion TB) that GPT disks allow.

Also, MBR only allows four primary partitions and requires an extended partition be built to hold other partitions called logical partitions. Windows operating systems can have up to 128 partitions on a GPT drive without the need to build an extended partition.

Another way GPT outperforms MBR is how easy it is to recover from corruption. MBR disks store the boot information in one place, which can easily be corrupted. GPT disks store this same data in multiple copies across the hard drive to make it much easier to repair. GPT partitioned disks and can even identify issues automatically because it periodically checks for errors.

GPT is supported through UEFI, which is intended to be a replacement to BIOS.