Easy way to backup/copy MBR under linux

1. get to know where the current MBR lies 

sudo /sbin/fdisk -l

the line tagged with an asterisk in between (aligned with column "Boot" ) indicates what you want.

2. read out and write into wherever you desire using dd

dd if=/dev/sda4 bs=512 count=1 of=/root/mbr.FAKE 

this is on the presumption that step 1 above gives you sda4.