The primary unit of data storage in a personal computer is the hard disk drive (HDD), which retains data even when the computer is off. Other options like RAM and external cache serve different temporary functions and are not main storage units. Therefore, the correct choice is the hard disk drive.
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The primary unit of data storage in a personal computer is the hard disk drive (HDD), which stores all data files and applications. Other options such as RAM, external cache, and media cards serve different functions and are not the main data storage unit. The HDD retains information even when the computer is powered off. ;