I put a 128 in my Shield and it formatted for me no issue. The device (Marshmallow update) will ask you how you would like to configure the additional storage. You can set it up like removable media or as additional device storage. It will explain each option to you as you go.
Update: The device was running VERY glitchy after the update to Marshmallow. Not sure how it ran before, the update came just after taking the Shield out of the box. So I did a factory reset, this killed the WiFi altogether, as in no signal or connectivity at all. So I did the factory reset over again. This time it was much better, at least until I started loading apps. I had set the 128g SD card to function as additional system storage. As the apps loaded and parts were added to the SD card, things would start running awful. So, one more factory reset with no SD card and the device is running great.
Still not sure I'll keep it now. It was to be an upgrade for my 32g Nexus 7 (2014). I really like the tablet and the 8" screen size. I've loaded all the apps I use and still have about 7gb to go. This along with Dropbox, Onedrive and Google Drive should workout fine.
Posted via the Android Central App