I have a 2015 12" MacBook. I've tried booting and holding D--and in addition to that I've tried option-command-D, option-D, and command-D. Nothing has happened other than normal boot-ups to the password screen, then nothing has happened after I've entered my password other than normal boot-ups. Nothing has been connected to my MacBook other than the power supply that came with it (and I've also tried all these holding-D iterations while disconnected from power). I've also tried these with my iOS devices powered off, just in case the wifi syncing has something to do with it. Nada. Have I missed something or have I done something incorrectly? (I've been trying to figure out if there's something wrong with the MacBook's battery by using Apple Diagnostics. I'm not sure if that's the proper tool...) Thanks for any input/help that y'all can provide!