6.0 Marshmallow: SD card storage does not show up when connected to laptop via USB?

Right, had this issue too on android 6...the Sd card or indeed android 6 just wasn't playing ball...there was no option to change storage destination, and no matter what I done, I could not figure it out. I was beggining to think that my new Sd card was corrupt, and I tried quite a few app2sd apps with no joy....so how was it solved? when formatting the sd card, it asks if you wish to write to sd by default...YES...the choose portable storage...NOT internal storage. when its all done, go to play store and install an app called "install apps on sd card-move". just tick box the apps and photos/videos and all the big files you have downloaded, and then click the sd card transfer pic up top, and it will move them to the sd card. none of the other apps are much cop, as they simply lead you to the android part of the "move/change" process which is missing... but this certainly bypassed that, and worked.
 
Okay. I Solved this one.

Migrate your data. :P That fixes the issue. I had a piddly amount of data, and thought "Pshhht.. not bothering".

Thanks LegoLink!

I had the issue with the SD not being recognized after "adopting" it as internal storage.

"Migrating" did the trick! Now it's just one big chunk of uninterrupted space. I could'a sworn I did that already.

Good thing, 'cuz I was about to turn green and go all Hulk on this phone!