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The Master Boot Record (MBR) will be created when you create the first partition on the hard disk. It is very important data structure on the disk. The Master Boot Record contains the Partition Table for the disk and a small amount of executable code for the boot start. The location is always the first sector on the disk.
The first 446 (0x1BE) bytes are MBR itself, the next 64 bytes are the Partition Table, the last two bytes in the sector are a signature word for the sector and are always 0x55AA.
For our disk layout we have MBR:
What will happen if the first sector has been damaged (by virus, for example)?
Lets overwrite the first 16 bytes with zeros.
When we try to boot after hardware testing procedures, we see just blank screen without any messages. It means the piece of code at the beginning of the MBR could not be executed properly. That’s why even error messages could not be displayed. However, if we boot from the floppy, we can see FAT partition, files on it and we are able to perform standard operations like file copy, program execution... It happens because in our example only part of the MBR has been damaged which does not allow the system to boot properly. However, the partition table is safe and we can access our drives when we boot from the operating system installed on the other drive.
What will happen if sector signature (last word 0x55AA) has been removed or damaged?
Lets write zeros to the location of sector signature.
When we try to boot now, we see an error message like "Operating System not found".
Thus the first thing if computer does not boot is to run Disk Viewer and check the first physical sector on HDD, whether it looks like valid MBR or not:
check, may be it's filled up with zeros or any other single character
check whether error messages (like you can see above "Invalid partition table"...) are present or not
check whether disk signature (0x55AA) is present or not
The simplest way to repair or re-create MBR is to run Microsoft's standard utility called FDISK with a parameter /MBR, like
A:> FDISK.EXE /MBR
FDISK is a standard utility included in MS-DOS, Windows 95, 98, ME.
If you have Windows NT / 2000 / XP, you can boot from startup floppy disks or CD-ROM, choose repair option during setup, and run Recovery Console. When you are logged on, you can run FIXMBR command to fix MBR.
Also you can use third party MBR recovery software or if you've created MBR backup, restore it from there (Active@ Partition Recovery has such capabilities).
What will happen if the first sector is bad/unreadable?
Most likely we'll get the same black screen, which we got when trying to boot. When you try to read it using Disk Viewer/Editor you should get an error message saying that sector is unreadable. In this case recovery software is unable to help you to bring HDD back to the working condition, i.e. physical partition recovery is not possible. The only thing that can be done is to scan and search for partitions (i.e. perform virtual partition recovery), and in case if something is found - display them and give the user an opportunity to save important data to another location. Third party software, like Active@ UNERASER, will help you here.