I was thinking of purchasing an external hard drive for my laptop for backup purposes. I'm looking for USB and 500GB or more.
Considering it's backup and I want it to be really portable, I'd prefer it not to have to be powered. What are the pros and cons of having it self-powered, such as by USB, versus having it powered by the mains? Does it run faster?
Not plugging it in to power is great when you're travelling -- on a plane, for example. I assume it is less prone to lightning/electrical issues.
So-called 'self-powered' USB hard drives are very convenient, but because of the power limitations associated with the Universal Serial Bus, only certain low-power drives can operate in this manner.
A self-powered USB storage device typically uses a 2.5-inch laptop hard drive. These drives are limited in size to approximately 250GB -- at the time of writing -- and don't require much power to operate. You should be aware, however, that they can drain your laptop's battery and may limit the power available to other USB devices. For best results, you should consider buying a powered USB hub.
Large 3.5-inch hard drives -- those with up to and above 750GB of space -- require much more power to operate reliably, hence the reason they're mains-powered. As you require a 500GB drive, you're better off buying one that's is powered by the mains. Be wary of large drives that claim to operate on USB power alone.