[Nexus 9 Q&A, Help & Troubleshooting] dsclee1:
First time poster, but thought I'd share how I managed to fix my problem, in hope that it helps someone else out.

I recently received the OTA Marshmallow update for my Stock Nexus 9 (unrooted and locked bootloader), but unfortunately it didn't work, and threw me into "bootloop" where it continually tried to apply the update but failed. First off I followed instructions to clear the Cache, which only made the problem worse, as all the tablet would do then was boot into the “swirling colour dots” and do nothing else until eventually the battery died.

After reading several forum posts I discovered that the usual solution is to flash via fastboot, however on a Nexus 9 you need to unlock the bootloader by activating "Enable OEM Unlock" from within Android. As I'm running a 100% Stock Nexus 9 I had never done this, so couldn't flash it.

Eventually though, after having an absolute nightmare trying to get it to work again, I found a working solution! Just use "Apply update from ADB" under Recovery, along with the correct update image!

A lot of what's in the steps below is likely to be known to most of the community, but I’ve tried to keep the steps extra simple so anyone can pick this up.

Steps:

1. Reboot into HBOOT (Hold down Volume Down + Power Button).

2. Press Volume Down to highlight Recovery, then press the Power Button to enter it.

3. Wait until the Google logo goes away, and you should see a picture of an Android laying on it’s back. Hold down the Power Button + Volume Up (very important to hold Power FIRST).

4. On the Recovery page it lists a few details about your version at the top. Note down the 6 character identifier for your device (which is in the format of 3 text characters, 2 numeric, and another text char). For me I was running 5.1.1 before the update, so my identifier was LMY48T.

5. Find the OTA update that upgrades from your current version to the next version. For me I was going from 5.1.1 to 6.0.0 on a Nexus 9, so I searched for LMY48T on Google and found that this was the OTA update needed: https://android.googleapis.com/pack...83199e.signed-volantis-MRA58K-from-LMY48T.zip

6. If you don’t have it already, download and install “Minimal ADB and Fastboot”: http://xdaforums.com/showthread.php?t=2317790 – when it’s installed let it run straight away, and leave the command prompt open you’ll need it in a min.

7. If you don’t have it already, download and unzip the Google USB Driver: https://dl-ssl.google.com//android/repository/latest_usb_driver_windows.zip

8. Copy the OTA update file you downloaded to the folder that Minimal ADB was installed in (usually C:\Program Files (x86)\Minimal ADB and Fastboot\).

9. Back on your device use Volume down to get to “Wipe Cache Partition”, and press the Power Button to activate it. If you want to be really sure this will work you should do a “Wipe data/factory reset” too, but be aware you will definitely lose your data.

10. Use Volume up to go to “Apply update from ADB”, and press the Power Button to enter it.

11. Plug your tablet into your PC (if it’s not already). You’ll need to go to your Device settings on your PC and you should see a new device named the same as your device or possibly just “Android”. Go to the Properties of the device and force the Driver to be the ADB Google driver from the Google USB Drivers that you downloaded earlier. (There’s plenty of other longer tutorials on how to do this if you’re having issues)

12. On your PC, in the Minimal ADB Command Prompt (that you should still have open from earlier) enter “adb sideload {NAMEOFOTAFILE}” e.g. for me this was “adb sideload ab835e17c874202049463ccd916f86fede83199e.signed-volantis-MRA58K-from-LMY48T.zip”.

13. Let it run. It took less than half hour for it to install for me, and I’m now the proud owner of a working Stock Nexus 9 with Marshmallow!

So why does this work but applying the stock images via fastboot doesn’t? As far as I can see it must be due to the “signing” of the files. The regular stock Google fastboot flash images don’t include signature information (as far as I’m aware) so they’re rejected as unsafe by the locked bootloader, whereas when you run the OTA update files through ADB sideloading signatures are on the OTA update files, and your device determines that these are safe to install.