External hard drive is the storage exchanged large-capacity data between computers, generally when choosing external hard drive, the factors you should considered is capacity, interface, the nature, volume and additional features, of course, price. The following specific factors should be considered when selecting it:
- Capacity.
The capacity of external hard disk is generally between 4.7GB ~ 40GB On the market, 4.7GB ~ 20GB is enough for normal use, because the external hard disk only is a transitional storage medium, there is no need to use much capacity. However, as the hard disk capacity and price is not proportional, high-capacity tend to have more cost-effective, and most of it less than 20GB is already eliminated, it is proposed that it’s better select it more than 20GB when you buy a new external hard disk, but if you buy hard drive box back and assembly it, that is another matter.
   2. Size.
 External hard disk composed of drive and hard disk boxes, the latter including the interface and control circuitry. There are usually two  size: 2.5 inch and 3.5-inch, corresponding to notebook and desktop computer hard drive. The size of 2.5-inch hard drive and the weight is smaller, more portable, but the price expensive than the 3.5-inch hard drive. Generally recommend selecting the 2.5-inch hard drive.
  3. Interface.
External hard disk is generally used USB interface (IDE interface has also been used early, but it’s inconvenient, you should open the case to connect it), you should pay attention to USB interface, it has two kinds of specifications: USB1.1 and USB2.0, the latter’s speed of data transmission faster than the earlier ones hundreds of times, basically have the same speed of exchange data with built-in hard drive, but the price is a little expensive. External hard disk with USB2.0 interface is the first choice.
  4. Purchase of finished goods or DIY.
You can select finished goods and used it directly when purchased external hard disk, but can also buy a suitable external hard disk box (which is not contain the hard disk, it contains the appropriate USB driver circuit and interface circuit, USB transfer cable, as well as an external power supply) in the computer market, then with 2.5-inch notebook hard drive, DIY your own USB external hard disk. HDD enclosure accompanied by an old laptop hard disk, the cost-effective entire external hard drives are higher.
You should pay attention to the distinction between size and interface specification when purchasing external hard disk box, and you can refer to the above description.
The attention of the problems when using external hard disk
(1).External hard drive connections:
USB connection cable of external hard disk is not only data transmission lines, but also it’s power lines, connection is too long will increase resistance and data interference, caused external hard disk does not work, so pay attention to: USB transfer cable as short as possible, in addition to the original connecting lines, should not be connected to extend line; second, you should choose  the USB interface (fixed directly on the motherboard interface) which behind the chassis, don’t use the front interface (which are connected by the board after a period leads to the front panel) of the chassis.
(2) External power supply of external hard drive:
The power of hard drives and data interfaces offered by computer’s USB interface when it working. However, when using external hard drive more than 10GB, relying solely on USB cable to offer power to the hard drive may be led to it not working properly due to electricity shortages, at this time it is necessary to use an external auxiliary power supply (DC power).
(3) Although external hard disk used USB interface, and support hot-swappable, but there is something you should pay attention to during use (Windows Me/2000/XP system): you must ensure that you have closed USB interface when unplugging the USB connections (mouse click on the date next to the USB interface sign, close interface), otherwise, a power outage may cause the hard disk drive damage when it in a high-speed operation!
