Intermittent flashing ? after Sierra Update I was required to update to Sierra to get Xcode on my machine. I did that yesterday. Currently it stands at:

Version 10.12.4
MacBook Pro (Retina,13-inch, Early 2015)
Processor: 2.7 GHz Intel Core i5
Memory 8 GB 1867 MHz DDr3

After this update,when my Mac goes to sleep or is shut down, about half of the time it displays the flashing ?, prompting hand-wringing from me. I start going through all of the steps for system restore, but often the SSD is not located during the system restore, and when I restart again maybe the mac finds the SSD and starts up normally. This is very annoying behaviour, of course.

I have had the flashing ? before, and I took it to an Apple service center in Trondheim where they held it for a month before returning it to me, with the diagnosis that the hard drive had unmounted physically and all that was required was re-inserting it. Great. Glad that took only a month.

I am tempted to believe that this is a physical hardware issue where the hard drive is experiencing intermittent hardware connection. But I've never experienced it shutting down in use, as you would think would happen if it were something like a loose connector, which makes me doubt this hypothesis. 

I asked this question on the apple forum, and I'm going to be reprimanded for the solution that I'm leaning towards: removing the back panel myself and staking the hard drive to the connector with adhesive. Taking it to a service center is unworkable for the following reasons: a) Because I'm currently on a Greek island in the middle of the aegean, and b) because I can't afford to lose my Mac for a month again for some tech to do what I want, which he probably won't because staking a hard drive isn't in the Apple book.

Any ideas here? I mention the Sierra update because of course, a lot of signs point to that, but the intermittent startups make me think that is circumstantial.
I'd do a NVRAM and SMC reset (in that order). Before following the steps below, ensure you have no external hardware connected and that you're using the built-in keyboard.

Reset the NVRAM

Reset the NVRAM on your MBP as follows:


Shut down your machine. Yes, a full shut down, not just logging out.
Press the power button and then press the commandoptionpr keys. You have to make sure you press these keys before the gray screen appears or it won’t work.
Hold those keys down until your MBP reboots again and you here the startup chime.
Let go of the keys and let your MBP reboot normally.


Note: When you log back in you may need to readjust some of your system preferences (e.g. speaker volume, screen resolution, startup disk selection, time zone information, etc).

Reset the SMC

There's a couple of ways you can reset the SMC on your MBP, however I prefer the following steps:


Shut down your MBP
Keep the power cable plugged in
Press at the same time shiftoptioncontrol (on the left side of the keyboard) and the power button
Let go
Turn your computer back on with the power button


After resetting both the NVRAM and SMC, use your computer to determine if the issue still persists.