Explaining Hard Drive Size - What is HDD Capacity, SATA & RPM?

Page content

Physical Aspects

There are a few things to remember about hard drives from a physical perspective. Obviously you need to know if you want it to be external or internal, but you also need to know how big it should be. Not how much data it holds but literally how much space it takes up.

Laptop drives are 2.5”, and desktop drives are generally 3.5”. You can fit a 2.5” in a desktop with inexpensive brackets. This is not only good for making use of an old notebook hard drive lying around, but some hard drives meant for desktops come in 2.5”, such as Western Digital’s Velociraptor. SSDs (Solid State Drives, discussed here), which are faster, lighter, more shock resistant, and use less power, but are very expensive, are usually 2.5” .

Another important physical feature, at least from a performance perspective, is rotational speed, or how fast the drive spins. Expressed in RPM (Revolutions Per Minute), the faster the drive spins, the faster it can find your data. It will use more electricity though, which may eat into your battery life on a laptop. 5400 RPM is standard for a laptop disk, 7200 (or 7.2K) RPM is on the nice side for a laptop and average for a desktop. 10,000 RPM is where it’s at for the fastest desktop disks.

Connection: Say Hello to SATA

External choices involve USB, which is good because any PC or laptop will have them, or Firewire or eSATA, which are faster than USB, but may not be available on every computer.

For an internal drive, unless you are stuck with older equipment that uses IDE-ATA cables, you will want to use SATA (Serial Advanced Technology Attachment) drives. SATA drives are easier to install and move data more quickly than the older IDE ribbon connectors.

SATA also allows for RAID (Redundant Array of Inexpensive Disks) which, if you have the right hardware, lets you configure several hard drives together for increased speed (RAID 0 or stripping), protection from data loss (RAID 1 or mirroring), or both (RAID 0+1 and 5)

There are some very fancy ways of connecting storage if you are working with servers and other industrial gear, but that really doesn’t affect what you will find in the desk- and laptop market. You may start to come across some SAS (Serial Attacheted SCSI) gear. You can plug a SATA drive into a SAS port, but not the other way around. That means your SATA drive will run on a system that has SAS connections, but a SAS drive will only run via a SAS connection.


I deliberately didn’t call it size because now we are looking at how much data the drive holds. This seems straight forward, but there is something to watch for.

Ever buy a new drive, get it home, and your operating system says it is smaller than it is supposed to be? The difference comes from us using a 10 digit number system, while the computer uses binary. If a drive says it has 100GB that means it has 100,000,000,000 bytes, in keeping with the conventions for prefixes indicating numbers (kilo, mega, giga, tera, and so on). The problem is that computers don’t consider a kilobyte to be 1000 bytes; their binary brains perceive a kilobyte as 1024, or 210. This is also true of megabytes, gigabytes and so on.

As a result your drive should have about (1000/1024)3, or 93% of the gigs listed on the box. A terabyte drive will have (1000/1024)4% of 1000, or about 910 gigs.

The International Electrotechnical Commission tried to clear this up in 1998 with the terms kibi, mebi, gibi, tebi and so on to indicate where the numbers are in line with the computer’s expectations; the **bi implies binary. It, sadly, hasn’t caught on.

That Covers the Basics

Now you know how big your drive should be, how much it should hold, and how you are going to hook it up to your computer. That’s enough if you want your hard disk to run and not fill up too quickly, but if you care about how fast it finds your data, read on. The next article will explain the numbers often used to determine how fast a hard drive is.

This post is part of the series: Understanding Hard Drive Specifications

Hard drive marketing materials and reviews offer a cornucopia of info. We help you find the important bits and tell you what they mean.

  1. Understanding Hard Drive Specifications
  2. Hard Drive Benchmarks: How Fast is Your Disk?