> However, if most of them aren’t, chances are you find a difference in the first block read, But just to read the first block, you need to open both files and

> However, if most of them aren’t, chances are you find a difference in the first block read, But just to read the first block, you need to open both files and move the heads from one to another at least once. If the files are placed in distant cylinders, this will be a significant amount of time (a few ms). At the beginning, fclones also processed files in each group matching by size together. But files matching by size do not necessarily live physically close to each other. Later fclones got a great speedup on HDD when I switched to processing the files in their physical order, regardless of their logical grouping. It is a bit more complex to do in terms of programming because in each grouping pass you have to "ungroup" the files to a flat list, sort the list by locations and group again, but it is really worth it (on rotational drives). You can read about this here: https://pkolaczk.github.io/disk-access-ordering/ https://pkolaczk.github.io/disk-access-ordering/