Why Do Magnets Mess Up Computers?
Here's a warning we've all heard: don't put magnets too close to your computer or you'll lose your data. While there is a grain of truth to this—magnets can mess up computer data—it's only true under certain circumstances. SD cards and USB flash drives are virtually immune to magnetic forces, as are the solid-state drives found in most netbooks today. What's (theoretically) vulnerable are conventional hard drives, though the magnet that would pose a threat is unlikely to be found around the house.
The Danger--in Theory
The hard drive, where computers typically store their operating system, programs and data, uses magnets to operate. A magnetic head moves over the surface of the hard drive and changes tiny magnetized regions, called bits, to say either “1” or “0.” These two numbers are the building blocks of all computer information. A magnet could theoretically throw things off by moving switches.
One Working Example
If you have an old floppy drive and disc around the house, you can see this in action. Write some documents to the floppy, then expose it to the most powerful magnet you have on hand. Reinsert the floppy; it'll probably be corrupted.
Why It Usually Isn't a Problem
Technology has come a long way since the floppy stored most data. Modern hard drives are built well enough to withstand magnetic force in doses one normally encounters around the home or office. While some powerful magnets can damage hard drives, they're typically only found in laboratories or government offices.