The drive was almost empty, but after complete defragmentation, free space is cluttered and not consolidated.
To consolidate the files try running a 'Defrag Freespace' or 'Defrag Freespace (allow fragmentation)' from Actions>Advanced, it may take a while to complete on a 1TB drive.
This explains the difference between the two options:
https://www.ccleaner.com/docs/defraggler/using-defraggler/defragmenting-free-space-on-a-drive
Actually I tried that, but with not much success. It only added fragmentation to some files.