In the recent weeks some users were unable to save to their usb disks. We have discovered that this is mainly due to lacking support for journalled HFS+ partitions in Linux (i.e. MacOS journalled, as it's called in MacOS' disk utility). Our advice is to forget about HFS+/MacOS formatting and go for ExFat. You'll also be able to access your disk on PC. We'll update the manual (v1.4 it will be) with this information.
The GR-1 currently mounts every partition present on an USB disk. That unfortunately means it also mounts the small 200MB EFI boot partition that's invisible in MacOS. We'll try to fix this in the new firmware and skip these partitions completely.
In closing, please use ExFat for everything, and please update your firmware. The 1.4 firmware should be out end of the week.