Bump. Having issues as well now...
Not sure when they started, but I'm actually having some issues as well.
Not only with my external SD Card [64GB Class 10 SDXC card], but with internal as well!
When I connect my HD to my laptop, and select "Mass Storage," the internal storage does NOT show up as a drive?!?! When I switch over to "MTP" device it works and I can transfer stuff just fine. This happens when I have AND don't have my external SD card in.
When I have my external SD card inserted, Mass Storage only shows the external card, but tells me I can't copy anything over? Same thing happens in MTP as well. Even from Android, with the external card in, I can't transfer anything back and forth or save anything to the external card from the internal storage.
I've tried factory resets and a full fastboot back to stock as well. Can't figure it out.
Anyone else having issues as well with internal storage not mounting as mass storage when connected to their computer?