Storage devices and secondary storage

Storage devices keep your files, apps and operating system when the power is off. Most computers today use one of two kinds: a hard disk drive (HDD), which stores data magnetically on spinning disks, or a solid state drive (SSD), which stores it in flash memory chips with no moving parts. SSDs are much faster; hard drives give more space for the money.

Why secondary storage is needed

RAM, the computer's main memory, is fast but volatile: it loses everything when the power goes off, and it is too small and costly to hold every file you own. So computers also need secondary storage. The AQA GCSE Computer Science specification defines it as any non-volatile storage that the CPU cannot access directly. Programs and files are copied from storage into RAM when you open them, and saved back when you are done.

Magnetic storage: hard disk drives

A hard drive holds one or more rigid disks called platters, coated in a magnetic material and spun at thousands of revolutions per minute. Each bit is a tiny region magnetised one way or the other. A read/write head on a swinging arm floats just above the surface, closer than the width of a hair: to write, it magnetises regions; to read, it senses which way they point.

Data is laid out in rings called tracks, divided into sectors. Before each read, the arm has to swing to the right track and wait for the right sector to spin round, about 10 milliseconds in all. That mechanical wait is why hard drives are slow at reading data scattered across the disk: at 10 ms each, they manage only about 100 scattered reads a second.

Solid state storage: SSDs and flash memory

An SSD stores data in flash memory chips. Each cell traps electric charge in an insulated layer, and the charge stays put with the power off, which makes it non-volatile. With nothing to move, any block can be read in about 0.1 milliseconds, so an SSD can make around 10,000 scattered reads a second, a hundred times more than a hard drive.

Flash cells wear out slowly: each can only be rewritten a limited number of times. SSDs spread writes across all their cells, so for normal use a drive lasts for years, but heavy, constant writing shortens its life. USB sticks and memory cards use the same flash memory.

SATA, NVMe and M.2

M.2 is the shape of the card and its slot; NVMe is the way an SSD talks to the computer over PCIe lanes. Many M.2 drives are NVMe, but some use SATA signals in the same slot and run at SATA speed, so check before you buy. The speeds come from the PC Components course.

HDD vs SSD

How long does a game take to load?

Loading reads the files from the drive into RAM, so the drive's read speed sets the time:

Take a 50 GB game, which is 50,000 MB. From a hard drive at 200 MB/s: 50,000 ÷ 200 = 250 seconds, over 4 minutes. From a SATA SSD at 550 MB/s: about 91 seconds. From a PCIe 4.0 NVMe SSD at 7,000 MB/s: about 7 seconds. Real loads also involve the CPU unpacking data, but the drive is often the biggest wait.

Try one with a random drive and size, then press Mark to check it in the course:

Cloud storage

Cloud storage keeps your files on someone else's servers, reached over the internet. As the AQA specification puts it, it still uses magnetic and solid state drives, just at a remote location.

• Advantages: reach your files from any device, share them easily, and the provider handles backups and extra space.
• Disadvantages: you need an internet connection, large files are slow to upload, you may pay a subscription for more space, and you rely on the provider to keep your data safe and private.

Common mistakes

• Calling storage "memory". RAM is memory; drives are storage. A phone with "256 GB" usually means storage.
• Saying SSDs use magnets. They use flash memory, which stores electric charge.
• Thinking M.2 always means fast. M.2 is a shape. Speed depends on whether the drive is NVMe and which PCIe generation it uses.
• Forgetting the units. Convert gigabytes to megabytes (1 GB = 1,000 MB) before dividing by a speed in MB/s.

Read next

Computer Science · Computer systemsRandom access memory (RAM): what it is and how it worksWhat random access memory (RAM) does, why it is volatile, RAM vs ROM and storage, cache, DRAM vs SRAM, and how to work out RAM bandwidth.