Are you being facetious? How does the storage medium on the other end know to send anything in the first place?
Here's a simplified version: The two devices communicate info in packets, with the OS on either end determining what needs to be sent/needs to be stored. The source is communicating to your computer what packets it's sending/sent, and your computer (you can call it your OS if you'd like) is constantly communicating back to the source what packets it received, and the source then sends more packets. It's like a give-take relationship. The source will only give when the client says it's ready to accept more. The OS knows when the hard drive has successfully written data to the drive, hence it knows when to ask for/receive new packets of info.
This is a very crude but accurate enough of an explanation that hopefully illustrates that while the hard drive doesn't "tell" the other end what it can do, it's still determining the absolute speed attainable (if the hard drive is the limiting factor).