Hard Disk Drive Internals

Hard disk drives are used in almost every computer. Most computer users never think of them until it is too late and they have lost some files. Since hard drives as other computer hardware are installed in a closed computer case, it is not surprising that an average computer user will have difficulty explaining how it looks like. Even more difficult it would be to explain what is inside the drive or how it works.

Hard disk drive is a device which stores digital data. The first drives were developed in the 1950s by IBM. The data is recorded on rotating platters with magnetic coating. Each drive has electronics which interfaces with the computer and controls writing and reading. This is the basic principle, but each drive’s component itself is a little sophisticated and fragile device which can fail. Failed hard drive can easily be replaced, but the data is lost unless you make backup copies. Despite the potential problem of data loss, hard disk drives are considered reliable, offer high storage capacity, random access, high speeds and because of low prices they are very affordable. A typical desktop computer hard disk can store 1 TB of data and costs less than $100.

However, disk drives could be made much more reliable but then the cost would be significantly higher. Current disks represent a delicate balance between reliability and price. For disk manufacturers it is much easier to make cheaper disks and replace them if they fail during warranty period. This approach lowers prices and forces manufacturers to further increase storage capacity and access speed. No hard drive manufacturer can afford disappointed customers so instead of high reliability they offer long warranty periods, usually 3 to 5 years. As a side effect of this “reduced” reliability and neglected backup copies, data recovery companies have a lot of customers.

Drive Components

The data is stored as magnetic recording on the platters. They must rotate to provide necessary movement between heads and magnetic media. Therefore every hard disk drive has mechanical and electrical components.

Open hard drive


Spindle holds circular disks called platters and rotates them with a fixed speed which needs to be very precise. Common rotational speeds are 3000 RPM (portable devices), 5400, 7200 or 10000 RPM (universal disks) and 15000 RPM (servers). Spindle motor is controlled by drive’s electronics and is the main source of the noise produced by the hard drive. In most modern disks fluid bearings are used to reduce the noise.


Platters are circular disks made of aluminum or glass and coated with ferromagnetic material. The data is recorded as changes in directions of magnetization. Platter surface needs to be highly polished to allow smooth hovering of heads above magnetic coating. Because of defects in the magnetic surface and because of possible contacts with heads, platters have damaged areas which are then marked as bad sectors on the logical level.


Write and read heads are responsible to write and read data from rotating platters. They create and detect changes in magnetization which is then converted to a stream of digital data.


Actuator is a linear motor that moves the arm with heads across the platter radius. It is consisted of a magnet and a voice coil.


Hard drive electronics has two main functions: to control the spindle, actuator and heads and to interface with the computer. One of standard interfaces is used to communicate with the processor which writes and reads the data from the disk. The role of interface electronics is to buffer and process the data.

Each drive uses some kind of logical model with addressable space to access the storage locations on the platters. A small embedded computer on the drive’s board takes care for this translation. Most disks provide also advanced algorithms to optimize head movements. They buffer all requests and process them in an order which reduces head moves and thus increases reading or writing speeds.

Because of imperfections and damages of the platter coating there are always some errors present during writing and reading. Hard drive electronics provides also Error Correction Codes which are used to detect and correct smaller reading errors. If the errors can not be corrected then this area (usually a sector) is marked as bad and removed from the pool of available space. In most modern drives this is done automatically by the onboard electronics. Disk drives also support SMART technology which provides information about all important drive parameters which determine drive’s health.

Hard Drive Interfaces

Every hard drive uses one of standard interfaces. Professional workstations and servers usually use drives with SCSI or SAS interface. Hard drives with these interfaces are more expensive than others but this higher cost usually means also higher reliability.

Drives for desktop computers use either parallel ATA (PATA/IDE) disk interface or SATA (Serial ATA) which is replacing the older parallel ATA interface.

Form Factors

Hard drives are manufactured in standard sizes or form factors. Desktop computers and servers use 3.5 inch hard drives, laptops use 2.5 inch drives while portable audio players and mobile telephones us 1.8 inch drives. There are also 1 inch drives mainly in the form of CompactFlash Microdrives. 0.85 inch disks are used in some portable devices.

Data Transfer Rate

Each hard drive has two data transfer rates. One is a property of the interface, the other is the real read/write transfer rate. For example, the transfer rate of SATA interface is 3 GBit/s but this rate only applies to the data transfer between computer and buffer on hard drive. Between buffer and R/W heads the data transfer rate is lower and depends on the rotational speed of platters.

Seek Time

Seek time is the time needed for the head assembly to move to the right track. It is measured in mili seconds. Additional time is needed to wait until the platter data sector is turned to the head.

Solid State Disks

Solid state disks are purely electronic devices and have no mechanical parts. They use semiconductor Flash memory to store data. To be compatible with classical hard disk drives they use the same interfaces. From the computer’s perspective they behave exactly as any hard drive with the exception of speed which is significantly higher. Solid state disks use standard hard disk form factors--typically 2.5 inch for factor.

Partitioning and Formatting

Before hard disks can be used they must be formatted. Formatting is a process where operating system or disk software determines and writes basic data structures to store files and folders. To format a disk you need to create one or more partitions. A partition is a logical part of the hard drive. Each partition is then formatted with one of popular file systems: FAT for removable media, NTFS for Windows systems, HFS for MAC OS and ext2, ext3 or ReiserFS in Linux environments.

Data Backup and Data Recovery

Because of the nature of how the files are stored and deleted it is possible to recover data in the case of disk failure or unintentional deletion. If the hard drive is physically damaged or broken it needs to be repaired first. This can be done in any specialized hard drive data recovery company. When the drive is working again the data may still be corrupted. The files can be restored by using a dedicated data recovery software which reads the disk data and searches for lost files.

Hard drives are used as storage of data. And for data storage used in personal computers, workstations and servers it is expected that the data will always be available. But because hard drives can fail computer users usually make backup copies of the files. Regular backup is the cheapest data recovery service.

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