Hard drives consist of a series of round metal plates called platters, also called cylinders. They are coated with an electromagnetic material which can support magnetic states that are capable of being electrically altered. This means some type of electrical signal can alter the magnetic polarization of various areas of the plates. The state of these polarized areas can also be sensed. Each platter can hold large amounts of data. There are several platters mounted on a hard drive. Between each platter is a head which is used to sense and modify the states of the platter. There are two heads on each platter.
Each platter has data stored on it in a specific pattern for read and write access. The data is organized into tracks which are rings around the platter. The distance the head moves into the platter will determine which track is read. A sector is a section of data in the cylinder. Different hard drives have different numbers of sectors, tracks, and platters. The total storage space on the hard drive is traditionally equal to:
Sector size times sectors/track times tracks/cylinder times the number of cylinders.
With more modern drives, however, to increase storage space, some drives have more sectors on the outer tracks than the inner tracks. This is because there is more physical room for data on the outer tracks. Therefore this method of calculating hard drive capacity may not be effective in the future.
Controller Interface Types
A hard drive is a mass storage device where your operation system is installed along with many data files. There are two types of hard drives with regard to the controller:
- IDE - Integrated Drive Electronics. A controller based interface. If your primary concern is low price with reasonable performance IDE is a good choice. It is still the most popular controller interface because of price.
- SCSI - SCSI uses a separate bus hooked to the system bus using a host adapter. It is a more expensive system than IDE, but is better built and has a great deal of flexibility. If you are considering running a server or high performance system, this is the best way to go. There are several types of SCSI interface, the primary characteristic being the width of the data transfer (how many data bits are carried over the cable at a time). The important item is to be sure you get compatible controllers with your SCSI device such as your hard drive or CD-ROM drive.
Most hard drives have three characteristics of main importance for performance.
- Size - The size of the hard drive is expressed in terms of Gigabytes which is roughly 1000 Megabytes. It is difficult to buy a drive less than 4 Gb today. Typical size are 8 through 20 Gb.
- Speed - The data output of a hard drive is primary limited by the amount of time it takes for the electromagnetic head to reach the data at specific locations on the drive. The primary factor of limitation is hard drive rotation speed. Common speeds today are 5400 RPM (revolutions per minute), 7200 RPM, and 10000 RPM. Considering price and performance, we currently recommend 7200 RPM hard drives.
- Reliability - The other performance factor that is worth considering is reliability. This is expressed as mean time between failure (MTBF) The higher the number, the better. Look for this specification on the manufacturer's specification sheets for each product.
- ATA - AT Attachment. This term refers to the type of IDE drive. Others are Microchannel (MCA IDE) and XT IDE. The ATA interface was used in the early 80386 based computers.
- ATA-2 - Refers to Enhanced IDE or EIDE.