Both are correct. When pronouncing HDD as individual letters, we say "an HDD". By contrast, when pronouncing the acronym as words (Hard Disk Drive), we put the article "a".

Both are correct. When pronouncing HDD as individual letters, we say "an HDD" because the "h" starts with a vowel sound (similar to "eich"). By contrast, when pronouncing the acronym as words (Hard Disk Drive), we put the article "a" (the consonant "h" is clearly pronounced).

A/an HDD uses an electromagnetic disk to store data.

Remember, the rule for using "a" or "an" is based on pronunciation, not spelling.

The most prominent characteristic of a/an HDD is its capacity.