I haven't actually worked out what nvflash -6 (as recommended by the readme) does. I'll report back after further investigation, but I'm going to be cautious, as I'm a bit worried about bricking the thing.
I wonder if it's been overclocked? I didn't get it new.
I did a quick dig on the internet to see if I could find something that helps. After piecing together a few sources it looks like
-6 allows firmware and adapter PCI subsystem ID mismatch
There used to be (and maybe still are?) options
-4 allows firmware and sub-vendor or hardware ID mismatch (for example when the VBIOS you want to flash is from a different vendor than the one who originally branded the GPU)
-5 allows firmware and adapter PCI device ID mismatch
To use all of these together one would enter
nvflash --index=1 -4 -5 -6 your.ROM
In case of a mismatch confirm the flash with 'y'
I personally haven’t used the nvflash tool, but if you are sifting through the utility’s help pages that’s probably the gist of what your eyes are squinting to see. BUT NOTE: the info I collected might be out of date! I am not certain all of those options exist in the latest version of nvflash.
If the utility is complaining about a single byte being wrong, perhaps what it really means is it encountered one of the mismatches described above? Seems like it would be safe to try flashing with -4 -6 enabled now that you know it will abort if the ROM itself fails to checkout. And if -4 is no longer available the command probably won’t even run.
If you add a -L to the command it should produce a log that may give you more explicit information about what is not working.
BTW, if you have a second computer on the network and the cMP is connected via ethernet you could trying SSH into it and that way avoid staring at tiny screen font on the cMP — you’d be staring at Terminal in the second machine which should be much easier to interact with.
Also, even if your used GPU was overclocked, that shouldn’t prevent flashing it with a different ROM.