I realise this is asked countless times but I genuinely believe my question is unique, please bare with me as I explain.
My Nexus7 (2013) is not working in fastboot mode, I connect my Nexus 7 and run the following:
$ adb devices
List of devices attached
07c15663 device
The Nexus 7 appears (once authorising adb), so I then reboot to fastboot (or bootloader):
$ adb reboot bootloader
The N
