A hard drive case is an enclosure designed to hold a hard drive outside of a computer. It is often known as a external drive caddy. The most common use of a hard drive case is to use a drive from an old computer on a new machine without the need to install it inside the machine.
The most important thing to consider when choosing a hard drive case is the connection type. The easiest to use is one with a USB connection, as this will work on nearly all computers. A USB 2.0 connection will give much faster speeds if there is a suitable socket on the computer. A FireWire connection, also known as an IEEE 1394 interface is also a fast and easy connection, though many computers only have one suitable socket, if they have any at all. Some hard drive cases use a Serial ATA, or SATA, connection, though usually the appropriate sockets are inside the computer itself, which severely cuts down on the convenience.
You should also check that the hard drive case is compatible with your operating system. In most cases, one designed for an older edition of an operating system will work with newer editions, often through a downloadable driver. With some lower-budget models, there may not be a driver for the latest operating system editions.
It's also important to check the case is suitable for the physical size of the drive you intend to use. In most cases, a standard desktop computer drive is 3.5 inches (8.89 centimeters) while a laptop or notebook drive is 2.5 inches (6.35 centimeters). The size refers to width, though this only covers the disk itself, meaning the drives are four inches (10.16 centimeters) and 2.75 inches (6.99 centimeters) wide respectively. The casing will be a little wider than this so that the drive fits snugly inside.
Another point to check is how the drive is powered. Most hard drives require more power than is available from the computer itself through a USB or similar connection. This means they will require a separate power cable, so you will need to make sure you have a spare socket within reach. If you are choosing between different models, it may be worth comparing the specifications of the respective power supply cables to see how common each is and thus how expensive it might be to replace if needed.
You may also want to check whether the hard drive case has an integrated fan. If it doesn't, the drive may get very hot, which could affect performance. If it does, the noise might be distracting. These aren't points that can really be covered by objective specifications, so it may be worth checking for user reviews instead.