Two possibilities:
(1) One of the USB ports are malfunctioning. Check you Mac with a smartphone, it if works then clean the port on your Glo. It might work, or it might be broken and in need of replacement.
(2) USB driver or the Glo's internal disk is corrupted. Kobo mounts a FAT32 disk, try another FAT32 disk. I don't think Kobo's internal memory is faulty, it would have given a different error then. But it might just be possible, check here to change it:
https://www.mobileread.com/forums/sho...d.php?t=209122 (there are unix tools to substitute, like dd, search for them)
Till you solve the problem or get a new device use COPS or Calibre Content Server to get books on your Glo. COPS is a little harder but would be faster and easier after setting up, Calibre Content Server comes build-in with Calibre. Just open it and browse [MAC'S LOCAL IP ADDRESS]:8080. You could see your machines local ip address in Network window, it would be something like this:
Wi-Fi is connected to Mercury and has the IP address 192.168.2.51
This is COPS working on my DigitalOcean VPS and in my Glo:
https://www.youtube.com/watch?v=GXSaK3RbaOQ (cover appears after half a minute or so)