you're not telling us much, so I'll tell you how I backup system disks for my own use:
- boot on a Linux live CD
- connect a USB disk with enough free space
- check which device is which (df, fdisk -l, etc ...)
- use dd to duplicate internal disk to usb disk. For example:
dd if=/dev/sda of=/media/external-disk/laptop.dd bs=1M
- you're done,
laptop.dd is a backup of your system disk. if you restore it to a blank disk, you get a system disk with all it's partitions, multiple boot, etc ...