"Connect USB Data Cable" message on fastboot screen preventing recovery/reset of Nexus 6

I have a Nexus 6 that doesn't boot after a failed Android M developer preview install (worked perfectly on my Nexus 5).

Booting into fastboot via power + volume down shows the device is unlocked but says "connect USB data cable". I believe "connect USB data cable" is the critical problem preventing any kind of recovery.

It is connected via the cable that I used to reboot it into fastboot successfully (and I've tried unplugging and re-plugging it in) so I think the cable is not the problem.

I'm guessing it's permanently bricked?