Despite some notions that SSDs or HFS drives don’t need defragmenting, I have often read and experienced myself that defragmenting your Mac every few years will clearly make it faster.
I had some trouble running iDefrag and would like to share a little trick I learnt. Basically it will refuse to run a full defrag, while your system drive is mounted. Restarting didn’t help. Here is what I did in the end: