Just in case, I decided to rule out the possibility that GPIO ports were somehow involved with fan control, the theory being that the GPIO ports were being exported, then unexported after they were used. However, the following command line script returned a string of “Invalid Argument” errors. In other words, no GPIO ports are available, or at least none within the numerical range that was probed, which means that ACPI is almost certainly being used for fan control
# for i in $(seq 1 1000); do echo $i > /sys/class/gpio/export; done
This is far from an isolated problem. In fact, simple research has been greatly hindered by an endless flood of people with the same complaint about their fan not working properly with Linux. Invariably, requests for help are followed by the same obsolete answers about trying lm-sensors, sensors-detect, pwmconfig, fancontrol, etc. Had any of these so-called internet “experts” done done a little research, they would know that the problem most often affects laptop computers, and the tools they say to use often don’t work on laptops.
As it turns out, the My Cloud PR4100 is basically a laptop computer with a bunch of hard drives inside, except that it doesn’t have a screen. It even uses a standard 19 volt laptop power brick.
Frankly, I’m running out of ideas, but I can’t shake this nagging feeling that I’m close… very close to finding the answer.