Check the current disks and what exists on them.
lsblk
sudo lshw -class disk
Create partition on disk
fdisk /dev/sdb
Choose n - for creating new partition, choose p - for creating primary partition (e for extended) and finally w to write the infromation on the disk. See help.
Next format disk to create proper filesystem. Check with mount command the filessytem type for your existing disks. So instead of ext3, you can choose that utility on below command.
/sbin/mkfs.ext3 -L <label for disk i.e. /backup-disk> <device i.e. /dev/sdb
Finally mount the disk on OS.