I have a properly formatted SATA HDD with the newest edition of Raspbian on it and am using a USB-SATA converter (with independent 12 v power supply) to connect it to my Pi 3. I successfully set the OTP bit and got a successfull USB flashdrive boot, so that checks out.
But the issue is that my HDD and USB-SATA get-up is too slow and doesn't respond within the allocated 2 seconds when the Pi boots. I read and read and reread the Official Raspberry Pi Documentation on GitHub and attempted to set the "program_usb_boot_timeout=1" at least 3 times, and still it won't work. I also read Raspberry Pi 3 USB booting , too.
Basicaly as superuser, I added "program_usb_boot_timeout=1" (without the quotation marks) to the /boot/config.txt on my Pi's SD card, saved the file, and rebooted.
Than I checked the status of the bit with "vcgencmd otp_dump | grep 66". No dice each time. All I got was "66:01009daa"
So what gives? what else can I do? Am I doing something wrong?