MBR tricks with Linux
The funny thing about the MBR is that it really isn't covered much in tutorials about installing Linux, however, it is a very crucial thing on your hard disk, and learning how to manipulate it is a very good skill indeed.
What is the MBR?
The MBR is an acronym that stands for Master Boot Record. It has two main functions: firstly, it holds the partition table of a disk, and secondly it can optionally contain executable code that the BIOS runs when the computer first starts up. If there's no executable code in the MBR, the BIOS simply moves on to the first bootable partition.
MBR structure
The MBR is located in the first 512 bytes of the disk (the first sector). It's organized as follows (taken from Wikipedia):
| Size (bytes) | Description |
| 446 | Executable code section |
| 4 | Optional Disk signature |
| 2 | Usually nulls |
| 64 | Partition table |
| 2 | MBR signature |
So what does this tell us? The most important thing to note is the overall size (512 bytes) and the executable code section (the first 446 bytes). Using this information you can do some fairly useful tasks.
Uses
Backing up a partition table
Probably one of the most useful things to do is to backup your partition table. This could be useful if you're planning to edit your partition table but don't want to lose the entire thing. To back it up, we're going to use the dd utility. To back up the MBR, enter the following at a Linux prompt:
dd if=/dev/hda of=/mbrbackup.bin bs=512 count=1
How does this work? if= refers to the input file. In this case it's your hard drive device, /dev/hda. (If you hard drive isn't /dev/hda for some reason, substitute it with the correct device.) of= refers to -- you guessed it -- the output file. In this case dd will create a new file in the root directory of the filesystem and save the contents of the MBR to this. bs refers to the block size (sector). Since the MBR is 512 bytes, you want to set it to that. Finally, count refers to the number of sectors to copy. There's only one MBR, so the count is set to 1. Now you should copy this file to an external device, because if the partition table gets damaged or erased, you won't be able to access the partition where you saved the file!
Restoring the MBR is equally easy:
dd if=/mbrbackup.bin of=/dev/hda bs=512 count=1
(This guide assumes you've already copied the mbrbackup.bin file from your backup media onto the current root partition.)
Erasing the executable code area
When Linux gets installed, chances are it installs something called GRUB or LILO. These are bootloaders that allow you to boot Linux, and if you've got other operating systems such as Windows installed, boot them as well. This works great for most people until they decide that they want to uninstall Linux and just go back to single-booting Windows. The problem is that removing the Linux partitions usually isn't enough, as many people have discovered. The friendly Linux boot screen refuses to go away even after wiping the partitions that it resided on.
The real problem is that the bootloader was probably installed to the executable code section of the MBR. Removing this is a little bit more difficult, but the dd utility again comes to the rescue. If you'll remember, the executable code section of the MBR is exactly 446 bytes. /dev/zero is a virtual device that continually produces zeroes (when needed), so this will come in handy. Enter the following to erase the executable code area:
dd if=/dev/zero of=/dev/hda bs=446 count=1
"But wait... how can Windows boot if the executable code area of the MBR is erased?" some readers may ask. Remember that it was mentioned earlier in this guide that the BIOS first checks the MBR for bootable code? If it doesn't find any, it goes on to boot the first bootable partition, and Windows contains its own boot code. Wiping the MBR is enough to get rid of the Linux boot loader and go back to single-booting Windows.
Erasing the partition table
If your partition table is for some reason toast and you just want to wipe it and start over, losing all your data, just write zeroes over the first 512 bytes of the disk:
dd if=/dev/zero of=/dev/hda bs=512 count=1
(However, if you wanted to write zeroes over the entire drive, you would just use dd if=/dev/zero of=/dev/hda.)
Conclusion
This guide is nothing new; these commands have existed for quite some time. However, it seems like many Linux tutorials neglect this topic, which is sad because it is very useful to be able to manipulate the first 512 bytes on your hard disk and understand how it all works.