Mac Pro 5,1 won’t display video, respond to keyboard commands, or seemingly boot any drives. possible firmware corruption of sorts, power button command not working as expected.

Your ARE doing something wrong.

You don't have a firmware problem.

<< it subsequently did something to the EFI. >>

that is complete Nonsense. Mac Pro firmware is VERY strongly backward and forward compatible, and the firmware is permanently stored on the processor, and you can't write older over newer whatever sequence you use. The firmware essentially can't be modified once upgraded. It does not change when you change drives, erase drives, or anything else, unless you change the processor module installed on the slide-out shelf.

Put away that ANTIQUE firmware CD and forget it ever existed. If you have installed Mojave, you are so far beyond the Firmware CD capabilities it's not even funny.

if you Mac won't start, the firmware is not the reason it won't start.

My response was calibrated to what and how you wrote.

You wrote on and on and on about your firmware problem. Without pausing or providing ANY white space in what you wrote.

Your post above makes no sense, because it is all based on the premise that you have clobbered your firmware. Because the text is so dense, it is impenetrable. I can not read it without losing my place and having to start over. I suggest you either:

• Run you experiments again, knowing that clobbered firmware is NOT the issue -OR-

• Provide a summary of what your symptoms are, and do not write more than five lines with providing visual break. Then your writing can actually be read, and re-read, and followed closely.

do the fans come on?

does it chime?

does the power indicator blink a code?

have you tried a power-based reset:

remove ac power.

wait an hour -OR-

press the power button for a quarter minute to discharge the power capacitors.

reconnect AC power.

attempt to start up.

The initial "chime" sound is generated in software when your Mac passes the Power-On Self Test. If the chime occurs and/or startup continues, your Mac is working. The screen should light up and draw a blank gray screen. Then on to the Disk Drive.

Accessing the Boot drive:

The solid Apple is not in the Mac's ROM at Cold start. The Apple logo can only appear when it is fetched in the first "blob" of software loaded from a 'magic' place on the boot drive, or re-run after a Restart. Then a whole lot of stuff is initialized, and the progress Bar moves part way across. After a cold start, seeing the solid Apple appear says your drive was able to produce the software that contains the Apple logo.

If a prohibitory sign appears at this point, it indicates some fundamental part of MacOS is damaged or wrong version.

Mounting the Boot drive:

The next step requires a lot of files by name, so the File System is initialized, and the Boot Drive is Mounted. If the drive directory is damaged, the drive can not be Mounted, so your Mac begins one pass of Disk Utility Repair. This will take an additional about five minutes. During this process, the progress bar may be extended, and will grow by an additional amount not seen on a routine startup.

at the end of that process (which should not take more than about five minutes), it will attempt to Mount the drive again:

-- if the drive Mounts, boot-up continues.

-- if the drive cannot be Mounted, your Mac can do nothing more, so it powers off.

-- if the process stalls, this may indicate you have Bad Blocks on your Rotating Magnetic Boot drive (if so equipped). The re-reading of Bad blocks can take a very long time (on the order of a quarter minute for each Bad Block).

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