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".
It’s an HDL. The abbreviation HDL is typically pronounced as individual letters. Since the first letter, the h, starts with a vowel sound (aich), it takes the article "an".
It’s an FDA-approved. The initial letter of the acronym FDA (the F) has a vowel sound (eff). Since the rule for using "a" or "an" is based on pronunciation, not spelling, we use "an".