After backing up my main system volume, I defragged it using iDefrag's "Compress" algorithm, and it worked just fine. But then I tried defragging a second time using its "Optimize" algorithm, and I got an "extents" error, and a message saying iDefrag coulnd't proceed. In order to fix the problem, I had to run the Apple Disk Utility twice, and then also Disk Warrior TWICE. The volume seems okay now, but I'm understandably a bit nervous about trying to optimze a second time using this application.