Hard Drives

Hard Drives

1. Hard drive disks (HDDs) are a form of magnetic secondary storage.

2. Physically an HDD consists of a disk (or multiple disks stacked vertically) called a platter (or platters). These platters are made up of ferromagnetic materials, allowing them to permanently magnetically polarised. Above each platter is an actuator, to which is attached a read-write head which is used to read and write information from and to each platter.

   • There are also various electronic components such as the HDD's Micro Controller Units, which include the electronics that control the actuator and allow the HDD to convert the analogue electrical voltage induced in the reading process into digital signals.

3. When writing to the HDD, the CPU sends digital signals to the HDD. The HDD converts these into analogue voltages, which create a magnetic field around the read/write head when appropriate. As the platter spins and the actuator arm moves grains of the ferromagnetic platter are permanently magnetised one way or another. This group of magnetised and unmagnetised grains approximates a series of ones and zeroes; binary data.

4. When reading from the HDD, the CPU requests a specific piece of data from the HDD. The platters then rotate so that the actuator arm is above the physical location of the data on the disc. The actuator arm then passes the head over the platter, which induces a varying voltage which depends on the magnetism of the point over which the head is passing. This voltage is converted into a digital signal and then sent back to the CPU.

5. The data storage structure for HDDs is defined in terms of tracks and sectors. Tracks are concentric rings that progress outwards from the centre of each platter. Sectors are defined by the areas created by the intersections of the tracks and straight lines from the centre of each platter to their edge. Collections of contiguous sectors (arranged in powers of two, ie: two sectors, four sectors, eight sectors) are called clusters.

   • When data is written to the HDD the HDD will try to write it into one sector. If this is impossible it will try to write it to one cluster to minimise the movement of the platter and actuator. If this is not possible it will write it to another sector and/or cluster. When data is spread across the platter(s) in this fashion it is said to be fragmented.