I'm still hanging my hat on your item 4: "A DIMM module performing below or at the lower end of the specification range (replace the DIMM)" Even if whatever arcane diagnostic routine a machine goes through pre-boot can't determine which version of macOS it's being asked to boot, can't it still determine that whichever version it's being asked to boot is beyond the DIMMs' capability?
During the Intel boot process one of the very first steps is the memory test and any condition tht would raise an alarm immediately terminates the boot process so nothing is loaded.
QUESTION: are you getting a single beep or double beep every five seconds? A single beep indicates NO RAM detected, a double beep indicates RAM is detected but it fails the RAM performance test.
I've worked on HDDs, SSDs, and DIMMs any number of times without having fried anything, which, of course, is no guarantee that I didn't do so this time, but I did handle the 8 GB DIMMs immediately after the 4 GBs "beeped," and I had no problem with them. (I'm sure there's some way to test DIMMs definitively, but I'm equally sure that the means to do it are beyond my ability to access.)
When I worked for Texas Instruments, even the software engineers were required to take annual ESD training. Part of that training demonstrated quite clearly that static discharges below the level humans can sense will fry a DIMM and even with good ESD procedures, accidents happen. As far as I know HDDs or SSDs are not subject to ESD damage. (getting hit by lightning excepted, but that would also result in significant physical damage as well.) Again when I worked for TI, we had equipment that could throughly test the entire performance range of DIMMs. Each test station cost 500 to 750 thousand dollars and that was half a century ago. I can't imagine what a DIMM test station would cost in 2022 but I know it is a bit beyond the range of amateurs.
And while we're scratching our heads, here's an interesting question: Assuming that the DIMMs haven't been fried - which I am - why did the machine boot with them before I changed the HDD, but not afterwards?
All I can speculate is "stuff happens."
I wonder if this qualifies as a "unified" theory of all that was wrong with Rita's MBP: I've already established - by both booting from my external and deleting a large chunk of data from the MBP's internal - that "offloading" "load" from the HDD improved performance, so I'll extrapolate from there and guess that the debilitated HDD diminished the OS's performance threshold by enough to raise it above the DIMM's performance threshold...self-cancelling errors, so to speak.
- There are any number of reasons and ways an HDD can negatively effect performance that would be mitigated by "off-loading" major chunks of data.
- Virtual memory swap files on that same HDD are critical to the operation of machines with small amounts of RAM
- Improving the peformance of the HDD will significantly improve the overall system performance of any computer, but especially on machines needing to make heavy use of VM swap files
- Could potentially make the difference in whether or not a given app can/will run
- Apple made significant changes in memory management techniques in Catalina including the switch to APFS for the boot drive and VM swap volumes
- APFS is optimized for SSDs and actually reduces the peformance of HDDs which exacerbates the previous negative performance factors
- under the circumstances off-loading files from the HDD would almost certainly result in improved system performance
- replacing the HDD with an SSD would have even more effect
- additional RAM would be even more efficacious
And I think I'm going to copyright that as a pretzel recipe!
More like a Gorgon's knot.
