optimising hdd

The pc with the old hdd would appear to be 15% fragmented so I set it to optimise and it goes through the motions of analysing, relocating and consolidating for several passes, finishes and tells me that it is 15% fragmented. I have repeated this procedure several times without success. This is build 9879.

I opened up Mini Tool Partition Wizard to see if if had a defrag option, it did not, and I noticed 500 Mb of unallocated space so I merged it with C and when it had finished and restarted the Optimise drives showed (C:)----Hard disk drive----Never run----OK(0% fragmented)