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.