You have officially been warned! There is a fatal bug in TWRP 3.0.2-1 that will brick your device. If you back up or restore EFS in TWRP your device will brick. Fortunately it isn't permanent as long as you can boot TWRP after the brick. There is a series of commands to run to unbrick the your phone if this has happened to you. For now you can unbrick the Nexus 6P. There will be methods for unbricking other devices later. For now just DO NOT backup or restore EFS in TWRP!
via XDA