Q: Defrag a imac

Have a look at the More Like This  section on the right...

But the short answer is there is no need nor benefit in Defrg for a Mac.

richard915 wrote:

 

My old imac is becomming slow.

Have a look here...

macworld.com/mac-troubleshooting-what-to-do-when-your -computer-is-too-slow.

Basically... Make sure you have enough Free Hard Drive space for your Mac to Perform as expected...

Rule of thumb is about 15 to 20 GB

And increase the RAM.

You're Welcome.

See:

Mac Maintenance Quick Assist,
Mac OS X speed FAQ,
Speeding up Macs,
How to Speed up Macs,
Macintosh OS X Routine Maintenance,
Mac troubleshooting: What to do when your computer is too slow,
Essential Mac Maintenance: Get set up,
Essential Mac Maintenance: Rev up your routines,
Maintaining OS X, 
Five Mac maintenance myths and
Myths of required versus not required maintenance for Mac OS X for information.