Adafruit Gemma, Macbook and the dim red light

My partner recently picked up an Adafruit Gemma as an intro to Arduino and embedded. It has not gone according to plan - it turns out there's a known issue where on some systems the Gemma will fail to enter the bootloader properly. The green power light comes on, but the red light stays dimly and constantly lit. If the reset button is pressed it goes out for a moment then returns.

Trying to program it anyway results in an error like

avrdude: Error: Could not find USBtiny device (0x1781/0xc9f)

The issue seems to be somewhat intermittent, too. Occasionally it works on her Macbook Air. It always works 100% of the time on my Lenovo T460 (Linux). So there's something platform related...

Not a great intro. The Gemma doesn't behave like usual Ardiuno systems - it doesn't expose a USB serial port, for example - so answers are also thinner on the ground.

But I think we found something.