Chosen Solution
I have a Mac Pro 2012 5,1. When I upgraded to Sierra a couple years ago it seems PCIe slots 3 & 4 stopped working. I later went to High Sierra and still those slots are not working. How can I fix this?
Boot from your original system installation DVD. Now see if the slots work. If so then you know it’s a software issue. If the issue persists you know it’s a hardware issue. This is one I have not see. I would blow it out very well with canned air, these things suck in a lot of dirt. If the slots work with the DVD, it’s a software issue. Now start up from your - wait a minute -Just how did you get it to High Sierra in the first place? When you went to Sierra did you do the firmware update?
