Ok, stupid question time.

I got my new Vantec 3.5" external enclosure today. I had a Maxtor 200GB HDD sitting around so I wanted to put it to good use. I will only use the HDD when I want to back stuff up. Other than that it stays in the box. Anyway, if I delete and add stuff to the drive, will I have to defrag it? Is that even necessary when I'm not running programs off the drive?