how to defragment imac

macOS's ability to defrag itself has been a built-in feature for over 20 years. If the elegant automated self-maintenance detects fragmentation, it automatically deals with it in the wee hours of the morning when most people are asleep.

It can wake a sleeping computer to do the tasks and then lets sleep resume. If the computer is turned off at that time, the routines run after the next startup.

Let it do what you paid Apple to include in the OS.

Please be more specific if you are having an issue with macOS.

FWIW macOS does not and has never require defraging like windows.

MacOS defrags itself little by a little bit on the run.

But using Carbon Copy Cloner to nuke & clone would seem to accomplish it...

http://www.bombich.com/software/ccc.html

Oh, on SSDs... never defrag.