Backup the MBR:

$ sudo dd if=/dev/sda of=sda.mbr bs=446 count=1

Back up the partion table:

$ sudo dd if=/dev/sda of=sda.pt skip=446 bs=64 count=1

Back up the byte signature:

$ sudo dd if=/dev/sda of=sda.bs skip=510 bs=2 count=1

Backup the extended partition table:

$ sudo sfdisk -d /dev/sda > sda.sfdisk