Hard drives are the primary storage device for data on most computers and laptops. On it is stored the documents you make, the photos you save, the games you play, and virtually everything else you keep on a computer. There are several things you should consider when trying to pick out a hard drive: Is the drive internal, or external? What is the RPM speed of the drive? What is the capacity of the drive? What is the physical side of the drive? Regardless of if you call them Hard Drives, Hard Disks or just HDDs you should always try to get the fastest and biggest drive you can within your budget -- with laptop drives being the only possibly exception. As far as performance goes, in general performance of a drive is a factor of three things:
RPM Speed - The faster the RPM speed is, generally the greater the performance is. Typical RPM speeds are 5400 RPM, 7200 RPM, 10000 RPM (10K RPM) and 15,000 RPM (15K RPM). Variable rpm drives also exist, with the most common being Western Digital Drives marked "IntelliPower".
Cache Size. All hard drives have a small dram cache on them they use to try to improve performance. A typical cache is going to be anywhere from 8MB to 128MB. In general, the larger the cache is, the faster the drive will be since speeds reading from cache are much greater than from the disk. A sub category of these kinds of drives are known as Hybrid drives or SSHD drives. Most commonly produced by Seagate they use a small SSD drive to supplement the cache on the drive and greatly increase speeds using a technique Seagate calls "Adaptive Memory". A similar effect can be done in software or at the motherboard chipset using a small ( > 90GB SSD) and a traditional drive SATA Drive.
Interface Type. The interface or plug on a drive is going to really determine how and if you can connect the drive to your PC or Laptop.
For external drives, in general the most important factor is going to be compatibility with what you have. The fastest external interfaces for drives are eSATA, USB 3.0 and Thunderbolt, however many PCs only have a USB 2.0 interface. For those machines picking a external drive that's either USB 2.0 or USB 3.0 (because it's backwards compatible) may make the most sense.
For internal drives, compatibility will also somewhat dictate what you can use. In general terms, if the PC was produced before 2007, there is a reasonable chance it needs an older style IDE / ATA connection. (Although some PCs had first generation SATA connections as early as 2003). If the PC or laptop was produced in 2008 or later, it is extremely likely that the PC uses SATA technology. We strongly recommend double checking the interfaces supported by your PC before purchasing a drive.
For servers, usually connections available are SAS - Serial Attached SCSI or the older legacy traditional SCSI ( Small Computer System Interface ) connection. Be advised, that traditional SCSI drives are now very rare, and those upgrading older servers are advised to purchase SAS controllers or replace the server with something that supports either SAS or SATA Drives. Those with SAS controllers usually have the flexibility to use either SAS or SATA drives as SAS is backwards compatible with SATA II.
SATA Drive Types and SAS
Drives using a SATA Interface typically come in three types: SATA 1.5Gbit/s ( SATA150 150MB/s speed), SATA 3.0Gbit/s ( SATA300 / 300MB/s ), and SATA 6.0Gbit/s (SATA600 / 600MB/s). As a rule, drives are normally backwards compatible one speed generation or more. I.E. If you have a SATA 300MB/s connector on your motherboard you can usually get by using a SATA 600MB/s drive, however the maximum speed will be SATA 300MB/s. Although no mechanical drive currently transfers at a sustained speed greater than SATA 300, it makes sense to use the fastest interface and drive available on your device as when reading from drive cache you will approach maximum speeds briefly (this is known as the burst speed of the drive). For a comparison of SATA with IDE/PATA see our article on the topic. As far as SAS goes, SAS also comes in two speed types: SAS 3.0Gb/s (SAS 300), and SAS 6.0Gb/s (SAS 600). As with SATA, similar advice applies as far as picking an interface -- however on a server you are typically using faster RPM drives and may be using RAID. This makes picking the higher speed more attractive, even sometimes if it means upgrading a controller card.
Hard Drive Size
Physically, hard drives usually categorized by their width, and then secondly for their thickness.
2.5" Hard Drives. Used mostly in Laptops / Notebooks, these type drives are very small and usually lower power than desktop drives. The trade offs you get with 2.5" drives are generally that the majority of them are 5400 RPM speeds, and the largest capacity ones you can buy are smaller than the 3.5" equivalent in price.
A secondary consideration with 2.5" drives is the drive thickness. The standard thickness used by most drives is 9.5mm, however thin and light notebooks or ultrabooks may require super thin 7mm thickness drives. At the same time, old laptops may use the previous generation 12.5mm, and servers that require 2.5" drives often can accept thicker 15mm SAS drives.
External Hard Drives based on 2.5" drives ( or 2.5" drives in 2.5" enclosures) have the benefit of usually not requiring an additional power connector.
3.5" Hard Drives. The most common size of drive used in PCs is 3.5". Generally, thickness is not a consideration with 3.5" drives and they may vary slightly in thickness depending on capacity or technology used.
External Hard Drives based on 3.5" drives will generally require a external power adapter.
Other Sizes. Other sizes of drive do exist, primarily 1.8" and the ancient 5.25" size, however they are very rarely used in today's market.
Hard Drive Capacity
The capacity available in modern hard drives has increased dramatically in recent years and price per GB has also come down a lot. As a rule of thumb, we recommend using a hard drive with a minimum capacity of 250GB for a new install of Windows or Linux. That said, given today's pricing the most common sizes purchased for that use are 500GB or 1TB. On desktops, it is fairly common for a second drive to be installed with a capacity of 2TB or greater for use as a backup location or to store media such as photos, video and music.