In What's a Computer? we told you a little about computer psychology - that it prefers to think in binary, using tiny "On" and "Off" switches called bits (only little kids can see them).
In every storage device except optical disks, those bits are microscopic magnets. When the poles of one of those magnets are flipped one way, this represents a "0" or "Off" bit; flipped the opposite way, this represents a "1" or "On" bit.
Put together arrays of billions of bits, then add in a means of reading or changing ("writing") the magnetic alignments, and you have a "magnetic storage device". Optical disks store bits in the form of dimples impressed by laser light (which, no doubt, is why CDs and DVDs are cuter than hard drives).
Except for the humble floppy disk or tape drive, modern magnetic storage devices generally have enormous advantages over optical media - they're faster, less error prone, and hold massive amounts of data - but they have one weakness which optical disks don't.
Due to the very fact that they are magnetic storage devices, they are susceptible to magnetism. As you probably know, this includes your bank cards.
Keep your magnetic storage devices at least 6 inches away from any source of magnetic fields - permanent magnets and electromagnets - including audio speakers, electric generators, motors, transformers, power supplies, and telephones, magnetized tools including paperclip holders and pocket knives, as well as sources of static electricity and X-rays.
Obviously, this depends on field strength. Many people don't know that the magnetic strips on security cards can be strong enough to corrupt the information on a bank card. Avoid storing them so they touch.
Static electricity can cause, not only corruption of magnetically stored data, but permanent damage to the miniature circuits in electronic devices since many of those circuits handle only very small electrical currents. Smaller currents can add up to big energy savings, but such components are more delicate.
X-rays can damage magnetically stored data. If you need to mail your data somewhere, it's best to send it on optical disks.
Though you are less likely to encounter them, there are two very strong sources of magnetism: "rare earth" magnets, used industrially and sold as novelties, and MRI (Magnetic Resonance Imaging) machines.
Keep all magnetic storage devices a minimum of 3 feet away from even a small rare earth magnet! They are so powerful that you can barely pull two of them apart. If they are allowed to snap together with any of your body parts in the way, they can draw blood. Otherwise they can fracture and throw shards and sparks. Use protective glasses when handling these things.
On the other hand, if you are disposing of old bank cards, or you have a broken floppy disk or USB drive containing sensitive data which you can't erase, a rare earth magnet will nicely obliterate the data.
An MRI machine is four times as powerful as the magnets which lift cars in junkyards. According to an AP news article, a Rochester policeman walked into an MRI lab while the machine was running. As soon as he entered the room, his gun was yanked away and slammed against the machine, firing a bullet into a wall. No one was injured.
To be fair, we should mention a little about optical disks. Keep them away from strong light, especially sunlight; and don't write on them with anything except a soft felt tip pen made for writing on CDs (or a Sharpie), and only on the label side.
The data, in the form of laser impressed dimples, is written on the inside surface of the label side. Minor scratches on the clear plastic side are much less a problem than scuffs and scratches on the label side.
If a disk has food stains or fingerprints on the clear plastic side, hold it by the edges under cool running water and use your finger to gently wipe from the center to the outer edges. Be sure to dry it gently but completely before mounting it in the drive.
Lastly, avoid putting dirty or dusty (sticky, wet, etc) disks in the drive. Also, buy and use an optical drive cleaning kit - it's cheap, easy, and well worth it.