Comment 2 for bug 1442416

I have the exact same issue, using an iMac11,3 kernel 4.18.0 ubuntu 18.04.

macfanctld only detects two 2 fans, (it fails to detect the HDD fan), and there is not way to control the speed of this fan that keeps ramping up to full speed, even though I am running Ubuntu from an external hard drive.