these clusters are small segments on the HDD, The Defrag process when runs starts at cluster 1 the beginning of the HDD and goes through to the end.
As it does this it knows which clusters on the HDD goes where. there are clusters that will not be moved, some are moved to the end of the HDD especially damaged clusters. I think I am using the word Cluster to frequent but thats what in IT terms I was taught.
Its a no really to your question as the HDD has the clusters put where they belong, yes they get shuffled about but only to optimise the drive.