A master boot record (MBR) is a type of boot sector, a data sector at the beginning of many types of computer mass storage. It is most common on disk drives large enough to be partitioned, hence it is not usually present on floppy disks or small thumbdrives.
The master boot record was popularized by the IBM Personal Computer. It consists of 512 bytes located at the first sector of a data storage device such as a hard disk. MBRs are usually placed on storage devices intended for use with IBM PC-compatible systems.
The MBR may contain one or more of:
Because of the broad popularity of PC-compatible computers, the MBR format is widely used, to the extent of being supported by computer operating systems in addition to other pre-existing or cross-platform standards for bootstrapping and partitioning.
If the boot record was corrupted, you cannot boot the computer successfully and you'll receive the errors such as "Operating System not found". Rebuild MBR feature will rewrite the Master Boot Record of the selected disk without destroying the disk partition table.
Step1, launch partition magic server software, click "Disk" > "Rebuild MBR".
Step2, select your Operating System from the drop-down box.

Click OK and back to main window, press "Apply" button on top left to proceed, done.
As original operating system cannot boot, you need to create partition magic bootable disc on another computer, and then boot from this CD/USB disk on your computer.
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