Macs seem to like to corrupt the Twiddler firmware when connected via USB. There is something funky going on in Apple's USB implementation that we have not been able to track down. We've talked to Microchip and they are stumped as well.
The hardest part is not being able to replicate the event reliably. There seems to be a random glitch that corrupts the firmware and only on Macs.
The frustrating part of this is that we have 2 versions of the firmware on the Twiddler. Factory Default and User Upgraded. This glitch wipes out the 1st block of storage where the bootloader and factory default code live. This is why all you see is the red light: the USB charging circuit controls that LED directly and does not rely on the microcontroller.
The fix is simple - just flash the microcontroller using the JTAG header. You would want to send it back here for servicing because the plastics get destroyed during disassembly. Its a 2 minute procedure and we replace the plastics with brand new ones.
If anyone has any ideas on why this is happening or can reliably replicate and document the procedure, we'd love to hear from you.