BrightKidz Library
Subjects
Three ways a computer stores files Three storage devices side by side. On the left a hard disk drive with its lid off, showing a round spinning disc and a swing arm reaching over it. In the middle a solid state drive, a flat board with memory chips on it and no moving parts. On the right a small USB stick with a metal plug and a loop for a keyring.

Exploring Computer Storage

About 5 minutes

Turn a computer off and on again and your files are still there. Something in the machine has to remember them while the power is gone — and there are three common ways of doing it, each with a different bargain behind it.

Hard Disk Drives

A hard disk drive stores information on spinning magnetic discs, a bit like a record player. A small arm called the read/write head swings across the surface, and to reach a particular file the drive has to wait for the right part of the disc to spin round underneath it.

That waiting is why hard drives are slow. It is also why they are cheap: the discs are just metal, and stacking more of them in is inexpensive. Because something is physically spinning inside, a hard knock while it is running can damage it.

Solid State Drives

A solid state drive has no moving parts at all. Information is held electronically in flash memory, so there is nothing to spin up and no arm to swing — every part of it can be reached in the same tiny amount of time.

That makes an SSD far faster and much harder to damage by dropping. It also makes it more expensive for the same amount of space.

USB Drives

A USB drive uses the same flash memory as an SSD, in a package small enough to lose. It is the one you can put in a pocket and carry to another computer, and it usually holds far less than the other two.

Which Is Which?

Each of these was said about one of the three above. Put them back where they belong — HDD is the hard disk drive, SSD the solid state drive.

  • Stores data on spinning magnetic discs
  • Cheapest way to store a very large amount
  • No moving parts at all
  • The fastest of the three
  • Small enough to carry in a pocket
  • Easy to lose

The Bargain Behind All Three

Look at what you just sorted and a pattern comes out of it. Nothing here is simply better than the others — every one of them trades something away.

Hard drives trade speed for cheapness. SSDs trade cheapness for speed. USB drives trade capacity for being portable. When someone tells you which storage to buy, what they are really telling you is which of those three things they think you care about most.

Inside a spinning drive, this is the arrangement doing all that waiting:

Inside a hard disk drive A round magnetic disc seen from above, with rings marking the tracks where data is stored, and a read head resting over the disc on an arm. the disc spins read head

The head can move in and out along its arm, but it cannot reach round the disc. For anything on the far side, it has to wait for the spin to bring it past — and that wait, thousands of times a second, is the whole difference between a hard drive and an SSD.