Most computers only have one hard drive. In some cases, you may see two drives listed in your file explorer, but this is probably because a single drive has been partitioned into two sections, one for your operating system and programs, the other for file storage. It is possible to add additional hard drives to many systems, particularly desktop computers. This can be done to increase the storage capacity, create data redundancy (using two hard drives to store exactly the same information, so if one fails you always have a working copy), or to enable your computer to boot into two or more operating systems.