External Hard Drives (Why don't some USB HD require DC plugs?)

Given the power limits of USB ports as Hawkeye22 outlined, it turns out that most 3.5" HDD units (the kind commonly used as internal drives) use more power than this; hence external HDD units based on those units inside them almost always need their own power supplies. On the other hand, many of the smaller 2.5" HDD units common in portable machines use less power and can be fed from a USB port alone, so there are lots of these units around that don't require an extra power supply module. Then there are the on-the-borderline units that require just a bit more than you can get from a USB port. Some of those rely on a USB cable with TWO connectors on the end. The second one MUST be plugged into a second USB port so it can draw from there the extra power it can't get from the first. Most of these are the 2.5"-based units, but there are a few 3.5"-based units like this.

There is a linked effect from this. The smaller 2.5" HDD units have a lower capacity that larger units, not surprisingly, AND there are many fewer of them made. The net result is that, as a general rule, the cost per GB of storage space is much higher for 2.5" HD's, and their maximum capacity is lower than you can get in 3.5" units. So, a larger unit based on a 3.5" HDD inside is likely to give you more space at a better price, but require its own power supply (usually supplied with the unit when purchased).

With the power limit in USB ports, I doubt you will find any portable external HDD's using only a USB connection that has a fan inside, also.