Back on the topic of the original poster, interesting article out today about a new "non-removable" virus for Mac:
http://www.zdnet.com/article/macs-vulnerable-to-virtually-undetectable-virus-that-cant-be-removed
Using a modified Thunderbolt adapter, researchers realized they could change the Boot ROM and infect that area with a virus.
So, friends don't let friends use strange Thunderbolt adapters!
Oh, and I've been a Mac user for about five years and never needed a defrag. Mac writes in smaller blocks than Windows, resulting in less fragmentation issues.
The only exception I've heard of is a friend who uses an older Mac Pro for professional video editing and special effects work. He regularly has a few hundred files over 1GB each in size.
(Perhaps as we move more to SSD, defrag issues will eventually be negated on PC's, too).
Cheers!
Chris