check whether disk signature (0x55AA) is present or not.check whether error messages (like you can see above "Invalid partition table".) are present or not.check, may be it's filled up with zeros or any other single character.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: When we try to boot now, we see an error message like "Operating System not found". Lets write zeros to the location of sector signature. What will happen if sector signature (last word 0x55AA) has been removed or damaged? However, the partition table is safe and we can access our drives when we boot from the operating system installed on the other drive. It happens because in our example only part of the MBR has been damaged which does not allow the system to boot properly. 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. That's why even error messages could not be displayed. It means the piece of code at the beginning of the MBR could not be executed properly. When we try to boot after hardware testing procedures, we see just blank screen without any messages. Lets overwrite the first 16 bytes with zeros. What will happen if the first sector has been damaged (by virus, for example)? The location is always the first sector 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. It is very important data structure on the disk. The Master Boot Record (MBR) will be created when you create the first partition on the hard disk.
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