Can't access boot options in MacOS Sierra When booting my mid 2014 13 inch Retina MacBook Pro, holding the option key to choose a startup disk doesn't work. I have just installed Windows 10 on an external SSD and the only way I can choose which to use as my startup disk is to change it from within whichever OS I am using, and my computer will boot into that OS until I change it again.

I tried resetting the NVRAM by pressing command+option+R+P on startup. All this did was boot into recovery mode. I want to be able to choose the startup disk by pressing option on startup but I don't know how to fix the problem I am having.
So this issue is more common than you'd think. To me what you're describing sounds like the issue where the keyboard is not initializing on time to recognize the full key sequence you're holding down. If you're getting to the Recovery Partition by holding command, option, P and R that means that the system is either only recognizing the command and R keys at that time (which happens to be the exact sequence to get to the Recovery Partition), or at the very least not recognizing the option key.

First I would try resetting the SMC in case it's a System Management Controller issue. If you've never done one it's actually pretty easy, you just have to make sure it's charged over 10% first. Shut down your Mac. Disconnect the power source. Press Shift + Control + Option on the left side of the keyboard, then press the power button at the same time. Hold all 3 keys and the power button for 10 seconds and release. (I do recognize that if the 'option' key is the issue, this reset may fail as well.)

If this works you should now be able to reset the PRAM as well as access the startup disk screen. 

If you still cannot reach the startup disk screen holding option, may I suggest testing with a usb-keyboard? (Not a permanent solution obviously but would give further insight as to what's going on.)