I have a mid '14 MacBook Pro 15" that will not boot due to a (1) typo I made in the nvram's boot-args, and (2) inability to reset the nvram. I suspect this is because my machine has a dead firmware battery, which it may have always had- I've always noticed that exhausting the main battery would fudge up the operating system's time and date upon next start up. Troubleshooting so far: Regular boot attempts and safemode boot results in a flashing "?" folder. Recovery mode boot attempt (requires my firmware password) results in a slashed circle and an endless cycle of verbose lines starting with "IOHDIXController: NOTE: administrator is creating non-ejectable disk image" and ending with "cannot mount root, errno = 19". See included picture of screen for the other 6 lines. I've also tried to reset the SMC (control-option-shift then power) as well as resetting the firmware/nvram specifically. The specific typo I made to the nvram's boot-args: (But this shouldn't matter, I should be able to reset the nvram and erase the nonsense, correct?) Inside of nvram's boot-args I set 'rd=disk1s1' which is an un-bootable partition- essentially forcing an un-bootable start up disk. Also, note: I have a firmware password set which shouldn't make a difference as I remember the password. All in all, I'm thinking it's a dead firmware battery which was long overdue for repair. What else could it be?