Except it was not about HDD priorities.
> To allow for two drives on the same cable, IDE uses a special configuration called master and slave. This configuration allows one drive's controller to tell the other drive when it can transfer data to or from the computer. What happens is the slave drive makes a request to the master drive, which checks to see if it is currently communicating with the computer. If the master drive is idle, it tells the slave drive to go ahead. If the master drive is communicating with the computer, it tells the slave drive to wait and then informs it when it can go ahead.
This pattern of truth flowing from one agent to another is called the master/slave pattern.
It is the simplest coordination pattern there is, which is why it was used to control people doing menial tasks. We agreed it's not a good idea and no longer deprave people of agency (as much), but it still remains the most simple coordination pattern.