I always feel "an USB drive" sounds wrong but "a USB drive"  sounds right.
		
		
	 
You are correctly going by what we are supposed to go by: the first sound of the next word, not the first letter.
"A university professor" vs. "An university professor"
"A union leader" vs. "An union leader"
"A unique problem" vs. "An unique problem"
"A USB drive" vs. "An USB drive".
All four examples correctly use "A" not "An" because the first sound of the next word is a consonant sound, not a vowel sound.
"A ugly post on ATOT" vs. "An ugly post on ATOT"
This time, the "An" version is correct because "ugly" starts with a vowel sound.