I got this kind of problem, it 's a driver issue. if you check about this Mac, you will probably see your graphic card is 0MB or not fully loaded. i had to boot the system with another boot loader with different USB. :crazy:
Can you help please to boot without USB. here is my disk list:
ast login: Tue Oct 4 22:34:37 on ttys000
Alans-iMac:~ alan$ diskutil list
/dev/disk0 (internal, physical):
#: TYPE NAME SIZE IDENTIFIER
I thought I was the only one who got this issue. I spent 5 days to see if I could find a bootloader for my PC. NO RESULT. I have to prepare for my oracle exam, I could not do anything...My El Capitan was working fine since I installed Sierra everything is a hell. I even tried all clove...