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

[Pixelation]

I noticed a lot of the external hard drive enclosures, that comes with or without the hard drive, requires a DC plug in.
Such as:

Fantom Drives G-Force 1TB USB 2.0 / eSATA External Hard Drive GF1000EU
http://www.newegg.com/Product/Product.aspx?Item=N82E16822204069

or

Rosewill RX-358-S BLK (Black) 3.5" SATA to USB & eSATA Ext. Enclosure w/Int.80mm fan
http://www.newegg.com/Product/Product.aspx?Item=N82E16817173042

Other custom made external hard drives do not require this DC plug in... and gets the power from the USB plug alone.

TOSHIBA 500GB USB 2.0 Black Portable Hard Drive HDDR500E04XK
http://www.newegg.com/Product/Product.aspx?Item=N82E16822149101

What is the difference between these two? Can they both operate with the USB power alone?
What is the DC plug powering? The fan? In that case, do you need the DC plug to operate the hard drive to transfer files?
Does it make it transfer faster? (I don't think it would since it's still transferring through the USB cable)

Help me out please
Thanks

 

[Hawkeye22]

USB can provide up to 5.25 volts and 500mA. If the device requires more power than this, it will need a plug.

 

[Paperdoc]

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.

 

[Pixelation]

Best answer selected by Pixelation.