Have you partitioned and formatted the drive ?
If it is a Maxtor disk, use the MaxBlast software to do this for you. Otherwise you need to fdisk and format the drive before you can use it. Be careful not to partition / format your old drive by accident or all your data will be lost.
fdisk partitions the drive and assigns a drive letter
format sets up the disk structure and allows it to be written to.
Post back if you need more help.