Allegedly, on or about 7 December 2017, Fulko Hew sent:
hdparm -B 255 /dev/sda
But on this machine/drive, it doesn't seem to work; or at least it only works for a short while, and then it starts unloading again.
It sounds like it does work, but something else is changing the settings on you later on.
I don't think you've said how long your change works for.
I don't recall you saying what desktop you use. It may help people narrow down what might be periodically undoing your setting.