system
Given the following post, respond as if you're commenting on Reddit:
human
How is it that my computer can load multiple files at the same time, if there's only one "needle" in my hardrive?
gpt
How can you see everything around you when you only have 2 eyes?

The read head only picks-up chunks of data at a time.  Any software that requests for an entire file in one go will lock your system, so they don't do that.  Since they only ask for chunks at a time it means any number of files can be read or written to the hdd by any number of programs... but if it so happens that a few programs decide to do some intensive read/write action you'll notice the OS being slow and / or those programs being unresponsive.  It's the same multitasking paradigm as the underlying operating system itself.  It's all just time-share.