#to copy or backup the MBR
MBR information is stored on the first 446 bytes of a disk, to backup this information you can save it with dd to another disk, or to a file.
dd if=/dev/sda of=/dev/sdg bs=446 count=1
dd if=/dev/sda of=/path/to/a/file.mbr bs=446 count=1
To restore the data to its original place using our backup file:
dd if=/path/to/a/file.mbr of=/dev/sda count=1