Defraggler seems to be pretty bad at finding loose (unconsolidated) files to fill gaps that appear in defragmented and consolidated space - granted, there aren't always available files to fill such gaps. But Defraggler's all or nothing approach means watching it move 100s of GB of data across one block at a time to adjust for small disk changes even if you did a full Defrag a day ago. Insanity!
Perfect consolidation is only necessary if you want to shrink a volume by the maximum amount. Otherwise, it’s not necessary.
I suggestion an option that allows Defraggler to have a certain amount of empty space between defragmented/consolidated files. You could either implement it as a slack space “bank” (of say, 1GB) or as a percentage of the total disk, ie. aim for 99% consolidation (instead of effectively 100%). Both could be configurable. This slack space would also be filled in naturally by smaller files ie. log files and such.
Also, please allow processing of multiple disks at once, and if possible, multithread the shuffle algorithm - the disk should always be the bottleneck here, never the processing.
I would definitely buy a pro version with these options.