So, this is weird... my 2010 cMP 5,1 was running fine on 10.14.4 and firmware 126.96.36.199.0 until last night. Last night I finally undertook the firmware update process to 188.8.131.52.0 and the install of the full 10.14.5, no problems. At the conclusion of it all, I booted my Mac up normally and confirmed that it is indeed running latest Mojave 10.14.5, and the firmware is upgraded to the 144 version. Then, since I planned to disable hyperthreading via the Apple supplied method, I tried to boot into Recovery partition. No go! I held the Command-R keys down from start-up through several minutes, and seemingly got no where. Eventually I just held the power button until it powered off. I repeated the attempt several times, varying when I initiated the keypress (at instant of power button pressed, just before tone, middle of tone), and how long I held it (a few seconds beyond tone, until the backlight of the screen came on at about the two minute mark, then several minutes beyond that). I think the longest I let the whole thing go in an attempt was ten minutes, though the longest I held down keys was around four minutes. Some random facts about the situation: - There is only one bootable disk in the system, the Mojave disk (SSD), and it is in a SATA slot, not on a PCIe card. There is a second disk in the system formatted as ZFS, but it should be a non-participant as far as the Recovery partition. - The computer has a Metal-capable RX 560 GPU. - On one of the later attempts I reset the PRAM, but not all attempts were with the PRAM reset. - On one of the later attempts I let it boot to full desktop normally, and then went in and manually selected the Mojave boot disk, just so that there wasn't any confusion about where to find the Recovery partition. - As I mentioned above, after about 2 minutes the backlight came on for the Apple Cinema display connected to the DisplayPort output of the RX560; the display remained black, however, and didn't give a block cursor in the upper left corner as it sometimes does during power-up and power-down. The second Apple cinema display is connected via DVI, and seemingly had no interaction at all. - My Mojave boot disk is formatted APFS, of course. - I know that I had a working Recovery partition under 10.14.4, since I booted into it without incident or holding down keys for insane lengths of time last week. - The keyboard is wired Apple USB, the mouse is wireless Apple trackpad gen 1. @bsbeamer mentions that one has to hold down the keys for a minimum of 3 minutes in this posting: https://forums.macrumors.com/threads/mp5-1-mds-mitigation-real-world-impacts-results.2182124/ That was news to me; I know that I held it down for more than three minutes on one of the attempts, but it probably wasn't before the PRAM reset. I'm wondering: should I pull the RX560 out temporarily, and put in the HD 5770 to access the Recovery partition? Do my dirty work of disabling hyperthreading, shut down, and then switch back to the RX 560?