- Disk with a k at the end is for floppy disks, hard disk drives, and other types of magnetic media [1].
Disc with a c at the end is for optical discs [1].
1 inch (in) = 2.54 centimeters (cm) = 25.4 millimeters (mm).

Analog vs. Digital

In analog storage, the information is tied to the storage. This means that it is not possible to copy analog information from one storage device to another with 100% identical results. In digital storage, however, the information exists independently of the storage. This means that it is possible to move digital information from one type of digital storage to another type of digital storage and have the information remain identical, although this is sometimes prohibited for legal reasons.


A floppy disk stores data via a flat, flexible, and circular magnetic device (the disk) inside a protective rectangular casing. A floppy disk is read by being placed inside a floppy disk drive (FDD). You can find out more about floppy disks by reading Wikipedia's Floppy disk article.

A hard disk drive (HDD), more commonly referred to as simply a hard drive, stores data via flat, non-flexible, and circular magnetic platters (the hard disks). However, unlike a floppy disk, the casing and drive for hard disks are one and the same. This means that a properly-operating hard disk drive will always have the disks inside, which in turn explains why a hard disk drive is often just called a hard drive.

Optical discs are flat, non-flexible, usually circular [2 page 17], and shiny storage devices with a hole in the middle for being placed in an optical disc drive (ODD).

Unlike the previous types of storage discussed, flash storage has no moving parts. Flash storage is mainly used for memory cards, USB flash drives, integrated flash storage, and solid state drives (SSDs). A solid state drive is flash storage in a hard drive form factor and can therefore act as a hard drive replacement. One extremely important point to be aware of is that even though flash storage doesn't have moving parts, flash storage, along with just about any other form of storage, can still stop working [3] [4].

Hybrid storage is a combination of hard drive and flash storage. This can take the form of a separate hard drive and SSD or a hard drive with integrated flash storage, the latter of which is referred to as a solid state hybrid drive (SSHD) [5].

Finally, it is important to point out that an operating system (OS) is typically stored on a hard drive, SSD, or SSHD.

Form Factors

The 5.25-inch form factor is used for ODDs in desktops.

The 3.5-inch form factor is used for hard drives in desktops.

The 2.5-inch form factor is used for hard drives in laptops. A 2.5-inch hard drive can have a drive height (sometimes called the z-height) of either 5, 7, 9.5, 12.5, or 15 mm [6] [7]. The 9.5 mm height is considered standard, meaning a 12.5 or 15 mm 2.5-inch hard drive probably won't fit in a laptop [6]. If the drive height is something other than 9.5 mm, it is typically specified somewhere on the packaging.

Additional form factors are discussed in Wikipedia's Hard disk drive article.


Older internal ODDs and hard drives use the Parallel ATA (PATA) connection. Newer internal ODDs and hard drives use the Serial ATA (SATA) connection.

Although SATA was technically introduced in 2001 [8], SATA products didn't start appearing until 2002 [9 page 6].

PATA uses one connector for 5.25-inch and 3.5-inch drives (40 pin) and another connector for 2.5-inch drives (44 pin). SATA, however, uses one type of connector for 5.25-inch, 3.5-inch, and 2.5-inch drives.

External drives use a USB connection and may also have a FireWire and/or external SATA (eSATA) connection.

Portable external hard drives are typically able to get all the necessary power from USB.

It's possible to have an internal drive act as an external drive by placing the drive inside an enclosure. One must get the appropriate enclosure for the type of internal drive that is being used.

Partitions, File Systems, & Recovery Options

In order for digital storage to store information, the device must use partitions, with each partition having a certain type of file system. Although most digital storage devices today are sold with only one partition, some potentially useful scenarios for having more than one partition are:

Storage vs. Memory

One last important point to cover is the difference between storage and memory.

To take an example, suppose someone has Microsoft Office Home & Student 2013. This edition of Office has Word, Excel, PowerPoint, and OneNote [10]. This means that at all times the programs are using up space on the PC's OS drive. Should any one of these programs be opened, that program will then get copied from the OS drive to the memory. When the program is closed, it will then get taken out of memory.

So, what it basically comes down to is this: storage is used for storing information on a permanent basis, while memory is used for storing information on a temporary basis. As a result, your storage dictates how much information you can hold while the memory dictates how much information you can work with at once. Increasing memory benefits those who use large programs and/or run several programs at once.



1. What's the difference between a "disc" and a "disk?". June 14, 2012. ©2013 Apple Inc.

2. Hugh Bennett. Understanding CD-R & CD-RW. Revision 1.00, Jan. 2003. ©2002, Optical Storage Technology Association (OSTA).

3. Andrew Ku. Investigation: Is Your SSD More Reliable Than A Hard Drive?. July 28, 2011 10:00 PM. Tom's Hardware. ©1999-2013 Bestofmedia Group.

4. Jeremy Laird. Best SSD: 10 of the top SSDs on test. Jan. 17, 2013. TechRadar. ©Future US, Inc.

6. How to Choose Between SSD, SSHD, HDD Storage for Your Laptop. ©2013 Seagate Technology LLC.

7. Achim Roos and Patrick Schmid. 9.5 Versus 12.5 mm: Which Notebook HDD Is Right For You?. May 22, 2010 12:00 AM. Tom's Hardware. ©1999-2013 Bestofmedia Group.

7. Internal Mobile Hard Drives Overview. ©2001-2013 Western Digital Corporation.

8. SATA FAQ. SATA-IO. ©2013.

9. External Serial ATA. Sept. 2004. ©2004 Silicon Image, Inc.

10. Compare Microsoft Office products. ©2013 Microsoft Corporation.