0
It is neither. In addition to input devices and output devices, there are devices called storage devices. The hard drive of a computer is a storage device: it contains the operating system and any data or program files used by the computer.
Although a hard drive both accepts input and provides output, it is not accessed directly by the user in either of these functions. A flash drive, CD, DVD, or external drive are other storage devices or media. In early computers, drives wrote to removable disks as CD burners and DVD burners do today. These types of media-writing hardware are considered output devices.
---
It's both, or neither, depending on how much of your hardware you choose to define as "your computer."
Since a device is basically something outside your computer that the computer communicates with, the answer to this question depends on what exactly you consider to be the boundary of your computer.
First of all, from the perspective of your computer's motherboard and in particular its central processing unit (CPU) chip, which actually do the "work" of the computer, a hard drive is clearly both an input device (because data can be read from it) and an output device (because data can be written to it).
On the other hand, if you define "your computer" to be "everything inside your computer's case" as much of the general public does, then an internal hard drive would be merely considered part of your computer as a place to store data (similar in this respect to computer memory, but slower), and only an external hard drive (one plugged in to the outside of your computer by a cable) would be considered an input/output device.
People involved with designing computer technology for a living (programmers, hardware designers, etc) tend to take the first view, while people involved with the business of making or using computers (computer repair technicians, computer salespeople, and most of the general public) tend to take the second view. Neither is really "right" -- it all depends on where you choose to draw the boundary between what is part of your computer and what isn't.
Some people that there are three types of devices, not two: "input devices", "output devices", and "storage devices." With that set of definitions, a hard drive would be neither an input device nor an output device, but would sit firmly in the third category, as a storage device.
