My 15" rMBP appears to be have been bricked by this firmware update. First time in ~10 years of being a Mac user that a firmware update has totally stuffed things up for me.
Stuck at grey screen (no Apple logo) at startup. Can't boot from the recovery partition or an external drive. SMC/PMU reset doesn't help. Unable to access internal HD using target disk mode.
Anything else I can try, or do I need to head to my nearest Genius Bar? Any help would be greatly appreciated!
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More information: Update seemingly failed to install at first attempt, and reappeared in my Mac App Store software updates after I rebooted. I then installed it again, which resulted in my current situation.