The reasons for rebuilding MBR

MBR (short for Master Boot Record), a data structure on the first sector of disk. It contains partition table for the drive and a small amount of excusable code for the boot start. If MBR of system hard drive corrupted / damaged, Windows can fail to boot and you may get messages, such as operating system not found. If MBR of second hard drive corrupted, you may fail to access it.