Hey I had this same problem using Defraggler running Windows 7 professional. I lost about 40gb of space after running the Defrag. I was looking at the options and saw in the tab, Action> Advanced> it gives you the option to defrag your free space so I ran this hopping this would help but I lost even more disk space. I also did the check disk for errors in the advanced tab as well and it came back with no errors. I am still searching for a solution, If I find one I will post it ASAP.