I can get a bit stubborn at times. I had modified a SystemConfiguration .plist file on my MacBook Pro so the Apple USB SuperDrive would work with it. The tweak, after restarting the Mac allows a bit more current through to the USB drive, allowing it to kick in.
My reason for doing this? Easy: the MBP is normally used in clamshell mode, held vertically in a cradle thingie jigger. Such an arrangement means the slim laptop DVD SuperDrive inside is forced to work vertically & it just plain ain't happy doing that, read/write error creep in or it gets all stuttery as gravity moves the optical disc too far from the laser for it to work properly.
But the Sierra update not only reversed that setting, it added copy protection so I couldn't modify the file. First I tried using TextWrangler to do the edit, that's what had worked previously but I could only save a copy & not replace the .plist file as I had previously done. So I tried doing it in Terminal, same result, copy only. As the file I'm working with is an essential part of the bootup process, I wasn't able to replace the file.
FU, Apple. Back to TextWrangler, make the needed changes, save to desktop instead. Then boot up from my Thunderbolt-connected TechTool Pro ProToGo diagnostic SSD.
Open the MBP's desktop folder & remove the word copy from the .plist file's name. Open the SystemConfiguration folder & replace the desired .plist file making sure to have a copy of the untouched original available elsewhere.
Replace the .plist file. Restart Mac to normal boot volume.
Lo & behold! The external USB DVD now powers up and can be used again with the MBP.