Hard disk drive


A hard disk drive (HDD) is a device with rotating magnetic disks -as the name states- to store data. HDDs come in various sizes (1.8", 2.5", 3.5") and speeds (4200 - 7200 rpm). There is always one in the computer. The ATA connection used to be the prevailing standard, but is now being succeeded by SATA.
Additional HDDs can also be connected externally using a USB, Firewire or eSATA cable connection. Such external HDDs are mostly used as fast backup, because with USB 2.0, eSATA and Firewire the data transfer rate comes close to that of an internal HDD.

More recent are Solid-State-Drives (SDD). These feature SRAM memory chips to store data. They have no moving parts and are a departure from the traditional rotating magnetic disks. Their main advantages -compared to rotating drives- are a very high writing and reading speed, very low power consumption (and consequently a low heat generation) and physical robustness. To this date they are far more expensive than traditional drives, which makes them only an attractive option for high speed applications.


The ATA or IDE Bus used to be the most common low-cost interface for Hard Disk Drives (HDD), CD-ROMs, DVD-ROMs, etc.. Depending on maximum transfer speed that is supported (in MB/s= Mbytes/s) we distinguish ATA/33, ATA/66, ATA/100 and ATA/133.
ATA is now succeeded by SATA - Serial ATA. This protocol offers a more efficient data transfer and higher transfer speeds. Also, the cables for SATA are much thinner and can be longer, which benefits the air flow for cooling within computer systems. Another notable difference is that SATA drives are hot-swappable, meaning that a SATA disk can be replaced without powering down and restarting the computer. SATA comes in three versions: SATA 1.5 GB/s, SATA 3 GB/s and SATA 6 GB/s.
In workstations and servers the SCSI connection is being succeeded by SAS (Serial Attached SCSI).

Performance parameters

Functional characteristics that PC manufacturers show in their folders are maximum transfer speed, storage capacity and (sometimes) access time (10-20 milliseconds). In consumer tests, the data transfer rates are also tested. Also, energy consumption is becoming an increasingly important criterium. A high energy consumption will increase energy costs, especially with servers that hold many drives and are powered on 24 hours a day all year round. Also, if the drive is not properly cooled, a high temperature due to high energy consumption will shorten the life expectancy of the drive.

Energy consumption

Presently, 3.5" HDDs use around 5-8 W during read/write, depending on speed. The 2.5" versions use less than 2-2.5W and as an external hard disk using USB they don't need an external power supply (max. 500 mAh/ 5V). The operating hours of the HDD depend very much on the type of use. In the BatteryMark test for a P-4 laptop the 2.5" HDD used on average 1.2 W during typical Windows operation. A typical 3.5" might use twice as much (2.4 W). Taking the latter as the typical standard practice (at 600 hours of annual use), the energy that can be attributed to a HDD is about 14-15 kWh/PC/yr.

Courtesy of the French Energy Agency ADEME, Future Electronics project. Expanded by VHK.



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