When you run out of storage space on your hard drive it doesn’t mean that it is time for a new computer. When you need to transport large amounts of information you don’t have to burn a stack of CDs that get out of order and require extra time to access. An external hard drive is portable and can implement your computer by a terabyte. They work for both PC and Mac, desktop and laptop. Every computer of any size can accommodate at least one portable hard drive. And some systems that have multiple ports can have several of them.
There are several types of external hard drives. The desktop model requires a separate power adapter and though it can be transported from place to place, is designed to be on top of a desk where it has access to power. Drives for laptops are designed to draw power through the connector cable whether it is plugged in or running on battery power. These "pocket drives" are small enough that they fit into the palm of your hand or in your pocket, purse, or briefcase. Some are so small you can stuff it into your pants pocket.
The capacity of desktop model external hard drives is available all the way up to 2 TB. Some models contain two units of 2 TB drives for a total of 4TB. That is enough space to hold all of the books of four large libraries. Laptop models are available in up to 1 TB, but most range between 640-500 GB. Solid state drives are also available, but mostly in laptop models. They are extremely expensive and cost is based on the capacity of the drive. The capacities of SSD external drives are limited to 256 GB. SSD offer some shock resistant attributes that can be handy in a laptop external drive, but much of the power and speed of an SSD is wasted if it is connected by USB 2.0 as the drive transfer rates are so much faster than USB is capable of.
External hard drives can connect to your computer via USB, FireWire, and eSETA. Even wireless USB and iSCSI are available, but rarely. Some companies market their drives based on speed. While 5,000 rpm is quite speedy, it will not make much of an impact on noticeable performance unless it is directly connected to the motherboard. Speed may of more interest to you if you plan on putting it in your desktop later.