There are four variables that can be used to classify a hard disk drive which are:
1. Form Factor (physical size)
4. Interface - the main differentiator
Internal hard drives were initially sized to match floppy drive sizes and have a compatible interface.
8 inch: (241.3 x 117.5 x 164.1mm) Disk drives this size will not have a SATA or SAS interface, since 8” drives were discontinued by the time they were developed.
5.25 inch: (203 x 164.1mm x 82.8mm or 41.4mm) There are two heights available: full height (82.8mm) and half height (41.4mm). Half height is more common, but both were discontinued by the late 1990s. 5.25” drives are unlikely to have either a SATA or SAS interface, since they were not introduced until 2003 and 2004.
3.5 inch: (461 x 414 x 203mm)
2.5 inch: (69.85 x 7 x 100mm) Originally primarily used for portable machines such as laptops, though from 2008 it replaced 3.5” drives in PCs. Higher capacity drives are 12.5mm high rather than 7mm.
Smaller drives, including 1.8”, 1” and 0.85” exist and were used in portable devices like mobile phones and memory cards, but this ceased in 2009 due to the popularity of flash memory.
The capacity of a hard disk drive is dependent on its form factor, year of manufacture and intended use (servers for example have much larger hard drives than those found in laptops).
Form Factor: 8” and 5.25” disk drives have smaller capacities than 3.5” and 2.5” drives because they were manufactured before the technology existed to store vast amounts of data. The table below illustrates the maximum capacities held by each form factor.
Full height 5.25”
Half height 5.25”
Be aware that these are the maximum capacities and actual drive sizes will vary widely. In part this is to suit the different budgets of consumers, but is also due to the technological abilities at the time of manufacture. For instance, 2.5” drives can reach 1 TB, but only since 2007, so no 2.5” drive manufactured before this will have a capacity this large. A brief chronology of size expansion may be useful:
1980: First hard drive disk has a capacity of 5 MB
1991: 2.5" drive expanded to 100 MB
2005: First 500 GB drive available
2006: 750 GB drives introduced
2007: 1 TB 3.5" and 2.5" drives released
2009: 2 TB 3.5" drives released
Despite 2TB and 1TB drives being available, most hard disk drives do not exceed 500GB.
The type and version of an interface used also has an impact on the maximum capacity. For example, the first ATA interface supports up to 137 GB, but version 6 (ATA-6) has a maximum capacity to 144PB, although disk drives don’t reach this size due to other technological limitations and the fact that it isn’t commercially viable. The SCSI interface could from the start support 2.2 TB and this increased to 9.44ZB in 2001, but again, hard disk drives are not actually manufactured in these sizes.
As a general rule all types of hard drive are compatible with the main operating systems. However, there are issues with larger capacity hard disk drives being compatible with older operating systems.
DOS systems generally cannot recognise drives larger than 8.4 GB and Windows 95 has a limit of 32GB. Windows 98 is restricted to 64GB, but this is not an operating system restriction, it is imposed by the Microsoft disk setup tools FDISK.EXE and FORMAT.COM.
The BIOS installed on a computer can also impact compatibility as pre-1998 BIOS cannot support drives larger than 8.4GB and pre-2002 BIOS cannot support drives larger than 137GB. However, this only affects ATA hard disk drives since the other types do not rely on BIOS for support.
The main difference between hard disk drives is the type of interface used. This connects the hard disk to the motherboard. The most common types are:
- ATA (also known as IDE and PATA)
- Serial ATA (SATA)
- Small Computer Scientific Interface (SCSI)
- Serial Attached SCSI (SAS)
All hard drives have two cables; one connects to the motherboard and the other to a power socket. The majority use the same power cable known as a Molex connector. There are several types, but the most common one used for hard disk drives is the Molex 8981 Series Power Connector. This has four conductors and the standard pin-out is yellow (+12 V), black (ground), black (ground) and red (+5 V).