Answer to "no chime / unresponsive to entering recovery mode etc after sudo nvram"I had the same issue. 2 days spent on it. The MacBook Pro started with black screen, no chime, no Apple logo. After some time it displayed the ? folder. I tried all the options, but no success, I eventually succeed by doing the NVRAM reset buttons (cmd-opt-R-P) for ALMOST 5 minutes and suddenly heard the chime (I press few time the power button to make sure the system started and continued to press the other 4 buttons). To conclude, I now have Catalina on my MacBook Pro retina 13' early 2015 128GB! I've upgraded to a Samsung 970 EVO SSD. However, I learned the hard way that it must be via macOS Mojave. You MUST have a bootable Mojave disk and once the system restart, you should erase the Samsung 970 EVO to reformat it and only then you can restore from backup. Hope it helps! BTW: Since the was issue was with Catalina I moved back to my original SSD.