I'm guessing I could replace the words "hard drive" with "random access medium" but let's be more specific here. Also for the sake of this question, let's not consider SSDs. Just plain old hard-drives with heads and magnetic discs.
I'd guess that the hard drive doesn't care about things as bytes and receives instructions from the OS on about where to move it's head, but that'd assume that the OS has knowledge of sectors, cylinders or whatever physical properties of the drive and it's stored data so that doesn't feel right.
So, how does this work?