Hi,
You'll need to identify the second hdd first. From the console, use dmesg | more and scroll down till you see the hard drives (ie. hda, hdb etc.)
As a generalisation, If your hard drives are IDE, they are called hdx where x is usually a for primary master, b for primary slave, etc). SCSI drives are usually sda, sdb, etc..
Assuming you have an IDE drive connected as Primary Slave, you probably have hdb instead of hdd in your system and so try using fdisk /dev/hdb instead.
Good luck !
Kelvin