External hard drives are great for storing photos, music, videos, and backup files. Not only can they can be used with a PC, but also with media devices to add streaming storage, and with Wi-Fi routers as cheap NAS solutions.
Off-the-shelf external hard drives often contain mystery drives inside and the enclosures aren't meant to be reused. Building your own external hard drive can sometimes be a cheaper, more flexible solution. And if you already have an old internal hard drive lying around, you can turn it into a cool external drive, for as little as $10.
Choosing your internal hard drive
Modern hard drives have SATA interfaces and are easy to identify because they only have 7 pins. If you're buying a new drive, it'll be a SATA drive. If you're reusing an old drive, it's possible that it's a PATA (IDE) drive, with 40 pins. Most enclosures support one or the other, so it's important to know which internal interface your drive has.
Size, height, and capacity
3.5-inch drives are used in desktop computers, while 2.5-inch drives are normally used in laptops. 3.5-inch drives offer higher storage capacities than 2.5-inch drives, so if you want 2TB of storage, a desktop drive is your only choice. However 2.5-inch drives are a better pick for portable use. While most 2.5-inch drives are 9.5 mm in height, some are 12.5 mm. Note the size of your drive before deciding on an enclosure.
7,200rpm and 5,400rpm hard drives are the most common rotational speeds for internal hard drives. 7,200rpm drives are faster than 5,400rpm drives, but they tend to run hotter and are less energy efficient than 5,400rpm drives. If you choose the faster, hotter drive, the enclosure's ability to keep the drive cool will be an important consideration.
Choosing your enclosure
Plastic vs. aluminum
Aluminum enclosures are more durable than plastic and inherently better at keeping drives cool. If you plan on running your external hard drive 24x7 or using a 7,200rpm drive, we suggest going with an aluminum enclosure. Plastic enclosures are generally cheaper than aluminum ones, but aren't as good at cooling. Occasionally, you can find a plastic one with a fan, but the drawback with fans is that they can get pretty loud, especially if they're sitting on top of your desk. On the other hand, if you're using a 5,400rpm laptop drive for portable use, a plastic enclosure is fine.
Choosing an external interface depends a lot on how you want to use your drive and how much you want to spend. A vast majority of enclosures are USB 2.0-only enclosures. Most media devices and routers only support USB drives. USB 2.0 is a good value choice and great for broad compatibility, but has the poorest performance among available interfaces.
USB 3.0 is currently the fastest available interface on enclosures and is backward-compatible with USB 2.0. The drawback with USB 3.0 is that it's not as common as USB 2.0, and Macs only adopted the format in mid-2012.
FireWire is more prevalent in Macs than PCs. FireWire 800 (EEE-1394b) is faster than USB 2.0, but slower than USB 3.0. For the best performance with a Mac, FireWire 800 is your best choice, unless someone releases an affordable ThunderBolt enclosure.
eSATA performance is better than FireWire and USB 2.0, but is the only interface that doesn't power the drive via cable; it requires a separate power adapter. eSATA drives have notorious connectivity problems as well. Unless you're dedicated to using this interface for some reason, we'd recommend against eSATA as your lone interface
Putting your external drive together
Putting an external drive together is fairly straightforward. You typically mount the internal drive onto the tray of the enclosure -- or sometimes the rear cap -- and then screw the enclosure shut.
That's it. After it's all put together, just plug it in to your computer and format it. For help on deciding what filesystem to use, Sharon Vaknin has a must-read post on.