MBR : Computer Hardware Buyers’ Glossary



MBR (Master Boot Record) is the first sector on a hard disk. It has been around since the first DOS (Disk Operating System) PC (Personal Computer) acquired the first hard disk circa 1980. It allows you to boot to several different operating systems, e.g. Windows 10, Windows 7, Ubuntu, DOS or to split your data up into several different drives e.g. C: D: E:. The original version allowed only 4 partitions (drive letters or operating systems). A kludge was added to permit an extended partition as one of the four primary partitions which could then be subdivided further. The MBR’s various fields were specified with 32-bit quantities, considered practically infinite in the day. Over the years, the MBR aquired some minor kludges to keep it working. For eample, the Bare Metal people invented the proprietary EMBR (Extended Master Boot Record) to allow more than four primary partitions.

If you have 512-character sectors and 32-bit fields, that lets you specify fields for a disk of up to 2 terabytes. An average size disk as of 2014-07-03 is 1 terabyte. You can see the crunch. Further, hard disks have 4096 byte sectors, but have to pretend to have 512. The lie causes a performance hit.

Intel could see the problem coming even back in the late 1990s. So, they decided the MBR needed an overhaul and invented the GUID (Globally Unique Identifier) partition table, (sometimes called the GPT (Globally unique identifier Partition Table)) that used 64-bit fields, which ought to hold us until we go extinct. It is also known as UELFI. Application programs will not notice the difference. Boot managers, partition managers and defraggers that run before Windows boots will need GPT-aware versions.

MBR Manipulation

If you want to manipulate the MBR, e.g. change the number or sizes of the partitions, you need some sort of boot-time utility called a partition manager to help you. Fiddling with the partitions is not for the faint hearted. I use a partition manager called Boot-It Bare Metal. It works reliably and is reasonably inexpensive though it is tedious to do even the simplest things. It handles MBR, EMBR, and GPT.


Sometimes viruses or ransomware corrupt the MBR making it impossible to boot from disk. You can boot from DVD (Digital Video Disc) or USB (Universal Serial Bus) drive and get to the command prompt and type:

bootrec.exe /fixmbr

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