Hi, Confused.

Hard drives put data wherever there's a physical space on any of the platters that's large enough to hold the binary in question. Once the data is broken down into small chunks called blocks, the file system (like FAT32 or NTFS) records where those blocks have been placed, and should they get shuffled around, where they have gone.

It's easiest to imagine that a hard drive is a big filing cabinet with a master index on the side that lists where every page of every document is stored. A hard drive might not put all the pages in the same drawer, but instead you can read the index and see what drawer each page is saved in.

A hard drive will attempt to store the entirety of a file on a single platter (it's faster this way), but it won't hesitate to store it on multiple platters if the need arises. As far as I am aware, a hard drive can only read from one armature at a time, though I'd be happy to see if I could get you an official answer from a hard drive manufacturer.

The biggest speed boost for a mechanical drive comes from increasing the drive's areal density, or how many bits of data can be stored per square inch. The higher the density, the less distance an armature has to cover to load an entire file.