MacPro 4,1 starting with grey screen after SMC and PRAM resets

I imagine. But it hasn't been left in place. I have taken it out as soon as I realised it was broken, and replaced it with the drive now carrying the system.

I did not know that. Now I connected both usb keyboard and mouse directly to the chassis, same the (otherwise) bootable usb stick, did an SMC and PRAM reset, no change.

Thanks so much for your patient help Grant, it finally worked!!

Interestingly enough, that worked, if only once. All worked normally, albeit slower in starting up than usually. Just to be sure I shut it down and tried starting up again. This second time I was back to plain grey, but still, maybe the fact that the hardware reset worked for one startup teaches something? Do you have any notion what governs the startup procedure and gets confused time and again?

(Btw. on your second last—nothing to be sorry about of course, you are helping, and I appreciate it!)

Good to know about the installer being erased, thanks.

I have a Time Machine backup of the startup drive. The ssd containing the previous system seems damaged, it doesn't even appear in disk utility anymore (regardless in which bay). I installed the system on a spare drive, works well if slow, but I see no way to use the existing Time Machine backup to restore the previous system (in its working state). Do you?