I tried it with a unit here that still has the old firmware. I upgraded it to the new beta firmware and then used it to format a 1.5TB Samsung drive that I installed in it. I connected the mini USB cable on the back to a computer, the EP350 prompted me if I want to go into the PC connection mode. I actually didn't do anything and just let the 10-second countdown end it self. After that, the player turned off, then turned back on but without any display through HDMI. About 30-60 seconds after that I got the prompt from my computer that a new device has been installed and is now available for use. I then copied some files back and forth without issue.
Can you try re-applying the firmware and re-formatting the drive again afterwards?
Micca Support Specialist