Unlike the SSD, an HDD consists of magnetically charged rotating disks that are divided into tracks and sectors. The HDD has a read/write head that is moved over the rotating magnetic disk. The information to be stored is therefore written to the disk by magnetization. The differently magnetized sections are also read out again with the read/write head.
Roughly simplified, the reading process can be compared to playing a record. Once a title has been selected on the index (the record cover), the reading head (the record needle) is placed in the discernible space between two titles (data tracks) on the record to scan the data (in this case, the music). If you want to hear another title, you start again from the beginning. With HDD, an interface and standardized connectors ensure communication with the computer environment. The HDD’s internal controller moves the read/write head to the addressed sections on the hard-magnetized storage disk. The data is retained on the HDD magnetic disk even without a power supply.
Unlike the SSD, the construction of an HDD requires high-precision mechanics. The magnetic storage disk lies in precision bearings and is set to a defined number of revolutions of 5,400 to 15,000 rpm by an electric motor for high-performance computers and servers. The read/write head is swiveled into the required positions by a separate drive. This also requires the highest precision, because a data track on the magnetic disk is only about 75 nanometers wide – which is tiny! The head itself “floats” on the airflow of the rotation 25 nanometers above the magnetic disk. By comparison, a human hair has a diameter of 300 nanometers. If the read/write head and magnetic disk come into contact, the infamous head crash occurs, which often leads to data loss.
The difference between a SSD and a HDD is quite significant. In an SSD, there are no longer mechanically moving parts, no rotating magnetic disk, and no read/write head. The data is stored in semiconductor cells. This makes use of the property of a semiconductor (solid state) of being able to retain – i.e., store – a state of charge once it has been assumed. The information to be stored is distributed among the many millions of semiconductor cells by a controller, which “layers” the data according to requirements and, if necessary, rearranges the data. The SSD also performs better than the HDD in terms of power consumption due to the elimination of electrically driven parts. In addition, weight and size measurements are lower.