Some desktop computers have multiple internal hard drives, and external hard drives are often used for additional storage or backup purposes. The first PCs had hard drives that were less than one megabyte in size, while modern hard drives may contain several terabytes of data. Most computers made in the 1980s, 1990s, and 2000s include an internal hard disk drive. The HDD is the most common storage device used to store data. Stands for 'Hard Disk Drive.' 'HDD' is often used interchangeably with the terms ' hard drive' and ' hard disk.' However, the term 'hard disk drive' is technically the most accurate, since 'hard drive' is short for 'hard disk drive' and the 'hard disk' is actually contained within the hard disk drive.