You don't necessarily have to buy a separate enclosure and drive and put it together yourself. Maxtor, Western Digital, Seagate et al all sell complete external drives. All you need to do is determine what type of interface the cable box provides for an external drive - USB or Firewire or both.
The majority use USB 2.0 but there are also many models that also support Firewire. I use a 300 GB Maxtor external drive to store music ripped from CDs and it is connected to the computer via Firewire.