TWRP 2.8.7.0 asks for password on Marshmallow (Nexus 9), cannot mount anything
Worked for me like this:


Boot into Fastboot
Flash Chainfire's WIP boot.img
Flash TWRP
Copy latest SuperSU BETA onto a USB stick
Use an OTG adapter to connect that USB stick to your Nexus 9
Boot into TWRP
Tap "Cancel" at the password prompt
Mount the USB stick
Install the SuperSU.zip from USB
Reboot


I didn't have to install stock recovery or anything else. It booted up just fine with all data still there.