As already outlined in the title, Defraggler permanently freezes when I select a drive from the list while defragging a different drive. I did this while trying to defrag two drives at the same time (I'm not sure this is possible - I was just experimenting).
I can consistently reproduce the bug with the following steps:
Click on D: drive
Click on "Defrag"
Click on V: drive
After doing this Defraggler freezes and never comes back. Task Manager still shows 100% D: drive utilization, so I'm guessing the defragmentation still continues in the background even when the GUI is frozen.
My Speccy: http://speccy.piriform.com/results/qsZqu2nT2TjHTNYFpOnspV1