Got a 17-inch 2011 MacBook Pro that won't boot. Tested with hard drive connected via USB to rule out bad SATA cable. Usually powers off or restarts, but when I try to boot recovery mode or a USB installer it blue screens (doesn't shut off or reboot tho). Strangely enough I can boot Apple Diagnostic Tools and the extended test completes without error. Given the year, I suspect it's GPU failure but less obvious than most I've run into.