[xubuntu] Software to entirely defrag a FAT32 hard disc?

I have an exteal USB hard disc in FAT32 format with some data on it.

I need to entirely defragment it.

(The hard disc will be read by a Playstation 2 with Open PS2 Loader on it. I only mention this to make clear that this hard disc needs to stay in FAT32 format, I am not interesting in changing the file system on that particular hard disc).

While I was still on Windows, I was using PowerDefragmenter for this purpose. The "normal" Windows defragmentation process, i.e. the defrag tool build into Windows, was not sufficient for this.

However I do not have Windows anymore, I only use Xubuntu 14.04, ruing in a VirtualBox under Mac OS X El Capitan.

Is there something that provides the functionality of PowerDefragmenter and similar software under Xubuntu? What program do you recommend? Can also be command line only, I am fine with that.

TL;DR: What software can I use to entirely defrag a FAT32 hard disc (for use with OPL on a PS2) using Xubuntu 14.04?