Not long ago, I upgraded the hard drive in my laptop from an 80GB 4200RPM to a 120GB 5200RPM. Not wanting to let my old 2.5" drive go to waste, I bought a rather nice USB enclosure for it.
Ah, but there was a catch. USB ports can only supply a maximum of 500 milliamps. The hard drive will need 550mA, meaning that the use of an AC adapter (or special USB cable) would be necessary under high load conditions.
I HATE those clunky AC wall adapters. I don't see why we should have to deal with them. Why can't we just have a centralized 12 volt power system in our buildings? We have it in all of our cars! That would be much more efficient.
The way this HD enclosure "solves" the power problem is to include a special USB Y-cable that can draw power from two ports. So one port handles power+data while the other just supplies power. This means that sometimes I'd have to give up two ports for one HD (or use an after-market AC adapter.) Not cool.
But I'm not here to complain. I'm here to awesome this problem out of existence. And we do that with some bits from Radio Shack, and, of course, a tin of Altoids.
I got the idea from ladyada's article
where she built a 2 x AA battery encased in a smaller Altoids gum tin. While it works and is super portable, it utilizes a power-management system that requires exotic parts (but you can just buy the whole unit minus Altoids from her.) Plus the output current is limited to about 200mA. Nowhere near enough power for what I need.