The effective speed of a hard disk depends not only on the revolutions per minute (RPM) or data throughput rate in KB/s (kilobytes per second), but essentially on how the files are arranged on the hard disk. Ideally, all files are located one after the other, each as a closed block on the hard disk. When reading the data, the read head of the hard disk therefore only has to pass over the data block once in order to read the complete file. In practice, however, this ideal state exists only shortly after a new hard disk has been formatted in Windows i.e. on the operating system used.
When deleting files, gaps appear on the hard disk, which later have to be filled by other data fragments. That’s how, over time, many files are distributed in several single fragments on the hard disk. Access to the files remains seamless, but will take longer than usual. When defragmenting, all fragments of a file are searched for and saved as a coherent block. In doing so, the contents of the individual sectors must be physically read.