Rather than defragmenting one drive after another, allow the user to select multiple drives and defrag them at the same time.
Yes this would require more resources, but as long as you are not doing anything intensive it shouldn't matter because defragmenting takes almost no cpu usage. Though it will takes a bit of ram. So in turn it will defrag all of the selected drives much faster than if it was to defrag each one at a time but your computer would slow during defraging.
Disk management snap in for computer management in windows allows you to perform operations on multiple drives simultaneously, for instance you can format one drive, change the letter of another, and do many other things at the same time or while other processes are already running. If you could do this in defragggler than it would make it easier to use the program, check one disk for consistency, defrag free space on another, and defragment a third drive rather than having to wait for it to finish it's current operation before you can do anything else with the program.