Came across this from a customer early last week... He had an external hard drive and had a power supply plugged into it, but it wouldn't work. I had a look at the power supply and it's rating (see the picture above RE: "OUTPUT") and it was only 6V, 300mA. This, I knew, was a problem because I've had extensive experience in power supplies and the like through my work, but the tip fit, so my customer plugged it in. I know that usually an external desktop hard drive (the bigger kind) will take 12V and 1-1.5A (1000-1500mA) Thankfully the external HD wasn't damaged due to incorrect voltage / amperage, but what if he'd plugged in a power adapter with a much higher voltage, say 19 or 24? It'll either cook it slowly or fry it straight out. The correct power for the extHD in question was 12v 1.5A as it turns out, so I managed to procure a 12V 2A power supply and off she went - "But the amps aren't right!" Amperage is a little different. If you use a power supply with more Amps than is necessary, that's fine, the device will pull what ever it needs and nothing more, however if you use an adapter with less Amps, then the device will try to pull more than what the adapter is willing to put out and you'll probably fry it when the device comes under heavy enough load that it tries to pull its max rating. The general rule is to make sure the V and A ratings are correct. The somewhat general rule is to make sure that V is correct and A is higher (on the adapter) than is needed. Sometimes though, you can't see the power rating on the device itself, so you'll need to find the model of device and do a bit of googling, or call someone to check it out for you.