I have a Mac Pro 2013 with an external GDrive. I also have win8.1 bootcamped on the internal drive. The Gdrive is partitioned for Mac (2TB) and Windows extFAT (1TB). I have lightroom for MAC installed on the internal drive, and it seems to operate fine. I regularly use the windows partition to store files I'm working on, and then switch back to OSx for processing. However, after importing some photos from the windows partition, working with them for a while, and then quitting, my Gdrive windows partition is completely destroyed (unreadable, unmountable)...tried to repair in diskutility, but no luck. I thought the first time was a fluke, but now this has happened two times!
exFAT format is not very fault tolerant:
Scroll down to below post at the above link for a possible recovery solution: