[Nexus 9 Android Development] KedarWolf:
Absolutely Everything You Need To Do Flash the Resurrection ROM Without Encryption

I edited this to remove some unneeded steps but added absolutely everything needed to the Resurrection Remix ROM unecrypted for anyone new to this sort of thing.

Go into 'Settings' then 'About Tablet' then tap on 'Build Number' until you have enabled 'Development Settings' Then go into 'Development Settings' and enable 'USB Debugging' if it is not already enabled.

Make sure you have the drivers installed and plug in the tablet using the USB cable that came with it to any USB port, Install TWRP and root your Nexus . I find the easiest way to do this is using WugFresh Development Nexus Root Toolkit. If Nexus Toolkit doesn't auto-detect your device just manually choose Nexus 9 and Android 5.0.1. then let it update everything it needs to. Then choose 'Full driver Install', and go to 'Step 3' and 'Google Drivers'.

Next install 'Root' and 'Install Custom Recovery' options. This roots your device and installs TWRP, SuperSU and BusyBox. Someone told me rooting this way isn't needed but some threads suggest installing BusyBox and SuperSU which I believe is the best way to be rooted for any ROM. Also it installs the latest TWRP which is nice.

Download the latest nightly Resurrection Remix, the GAPPS from the beginning of this thread and The ElementalX-N9 kernel. I haven't tried any other kernels but any one the disables encyption should work. Put the files you downloaded on your device then boot into recovery by shutting it down, hold the 'Volume Down' when restarting it and choosing 'Recovery' when the boot menu comes up. Alternately in the ' Developer Options' you can enable 'Advanced Reboot' then just hold your power key down, choose 'Reboot and then 'Recovery'.

First go to 'Wipe', 'Advanced Wipe' and wipe 'Dalvik Cache', 'System', 'Data' and 'Cache', NOT 'Internal Storage' though or you'll not have anything to flash and softbrick/bootloop your Nexus 9. Then flash Resurrection Remix followed by GAPPS then the ElementalX-N9 kernel. The Advanced Wipe options are the best way to get ready to flash custom ROMs and you'll have a much lower chance of any trouble after the flash if you do.

When everything is installed hit the back button twice to go to 'Reboot' on the TWRP home screen, then boot into 'Bootloader', chose 'HBOOT' then 'Factory Reset'. DONT just choose 'Reboot' on the screen right after the flash, you won't have the 'HBOOT' option, it'll just reboot and be encrypted, you need to hit the back arrow a few times to go to the TWRP home screen, 'then 'Reboot' and 'HBOOT' and 'Factory Reset' etc. If you just factory reset in TWRP it will still be encrypted as your it doesn't wipe it as needed.

It may take some time to boot. When it boots I have the 'Encrypt' option now in the 'Security' settings in 'Settings' which means the device is not encrypted.

You have to tap on 'Build Number' and enable 'USB Debugging' again and re-root using Nexus Root Toolkit as before. Should be no need to install the drivers again though.

Peace out.

P.S. If you want to try different settings in the ElementalX kernel just flash it again and choose the settings you want, you don't need to wipe your tablet in HBOOT or TWRP again to be unencrypted. Might be a good idea to boot into 'Recovery' and go to 'Wipe' 'Advanced Wipe' and wipe 'Cache' and 'Dalvik Cache' as many threads say to do that after installing a kernel.