Recently, I experienced a communication failure talking to somebody about a "hard drive" until I could actually show the person what I was talking about.
"Oh, a hard disk! I thought you were saying something else, like
'hard drive.'"
"Yes, I probably did say hard drive."
What difference is there between these two, if any?
The title of the Wikipedia article uses the less concise "Hard disk drive" which I've also used, and could be disambiugated in this question/answer as well.
Here is a Google Ngrams view of the two terms, showing that "hard disk" used to be more common but "hard drive" is increasing in popularity and the edge that "hard disk" has now is mostly attributable to the fact that it comes first in "hard disk drive."
Cross-link to related question: Etymology of the use of "Drive" to refer to a digital storage medium
Best Answer
Hard disk refers to the data storage elements themselves.
Hard drive and hard disk drive refer to the data storage elements plus all the electronics that support, or drive, the reading and writing of data to/from storage. If, for example, the power electronics were removed, you'd still have a hard disk but not a hard drive.
The difference between hard drive and hard disk drive is that the former is more concise.
Edit (thanks to Misneac's comment!): The distinction becomes clearer when you consider the distinction between a floppy disk (something people used to carry around) and a floppy disk drive (a part of a not-so-portable computer that one would put a floppy disk into) or the short form floppy drive which apparently became more popular than floppy disk drive around 1991: