How to copy master boot record (MBR)

MBR is divided into 3 sections
1) Bootstrap . 446 bytes
2) Partition table. 64 bytes
3) Signature. 2 bytes


To make a backup:
dd if=/dev/sda of=backup bs=512 count=1


To restore only the bootstrap section, the target disk has a different partition table. 
dd if=backup of=/dev/sdb bs=446 count=1