Kobo's internal memory is in ext4, OS X can't read that. But user accessible memory should have been mounted, it gets mounted in my internal microsd card. There are Kobo Glo microsd images in development forum here, try one of them with a *new* microsd card. You can use Disk Utility or these tools:
https://wiki.archlinux.org/index.php/Disk_cloning (I recommend dd, it is best tool for disk cloning and available in OS X)
There is a problem with your microsd, but I can't say more without more data.