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My Nexus 7 (2012 Wifi) got stuck in it's boot screen (4 circles animation) after the legit automatic OTA update of 4.3 to 4.4 (KitKat), and wouldn't even let me switch it off ...
To fix this issue, download Nexus Root Toolkit (free) and follow these steps:
1) Power Down and go to recovery Mode
If you can't seem to switch off you Nexus, keep the power, volume UP and DOWN buttons pressed simultaniously for up to 20 seconds, and it wil power down. I counted 19 seconds before the shutdown, others claim up to 20 seconds. The "broken Android" picture will appear ... you're Nexus 7 is now in recovery mode.
2) Start Nexus Root Tolkit and do a factory restore
After starting Nexus Root Toolkit you'll be asked what model Nexus you have and what Android version you're running. If not sure about the andorid version, simply pick *.* (last in the list).
Next, connect your Nexus via the USB cabe to your computer and wait a few seconds so Window scan recognize your Nexes.
Choose now the option "Soft bricked / Bootloop" under "Back to stock".
Next, click "Flash to Stock and Unroot" if your Nexus is showing the tipped over Android, and click "OK".
Nexus Root Toolkit will pick the latest version suitable for your Nexus automatically (at the time of writing this: NAKASI-GROUPER: Andrid 4.4.0 - Build: KRT16S voor de Nexus 7 Wifi van 2012), make sure it's selected and make sure "Automatically download + extract the factory image selected above for me" is selected as well and click "OK". The download process will now start.
After download has been completed, the download will be verified and extracted. A message will appear asking you if you're ready, click "OK" and a DOS window will open allowing you to follow the progress ... be patient. Messages about missing .sig files can be ignored. It took about 3 minutes on my setup.
After completion the Nexus will boot and might even look like it's boot loping again - be patient this can take up to 10 minutes ...
After that: you're Nexus is free again ...! 
does this work on the 2013 nexus v.2
I only have an older 2012 model, but looking around I see that the 2013 model appear much more prone to this issue (and with some users the problem keeps returning).
Some people claim that it might be hardware related (swap it for a new one), others claim that it possibly software related (after re-installing apps the problem resurfaces).
Most people get away with a recovery boot (press Power + Volume Down on startup, opens the bootloader, scroll to Recovery Mode using the volume keys and pressed Power to select it and boot into recovery). But in my case I couldn't even switch the Nexus off (had to wait for the batteries to die).
I did some more searching and found that the trick described actually should work for the 2013 model, but I have no means to test this.
Granted, when I tried this trick, the first 2 times failed on mine as well ... I suspect drivers issues or operator-error.
Keep in mind though: the selected firmware will be different, since the 2013 is a very different device compared to the 2012 model.
The Nexus Root Toolkit though should be selecting the right version. Key points to look at:
- Make sure it's a Nexus 7 firmware
- Select Wifi vs 3G model
- Pay attention to 2013 vs 2012 model
- 4.4 or higher for newer Android versions
There is also a FAQ for the Nexus Root Toolkit which tells me that it should work for a 2013 model.
After doing all instructons to try to get my Nexus 7 working I finally get the message:
ADB device was not found. In order for this to work USB debugging needs to be enabled on your device etc....
How can I do that when my device (Nexus 7 32 GB G3 2012 with bootloader version 4.23) is blocked and stuck on the 4 coloured circles that are shown till battery is empty! My Nexus just suddenly stopped and after restarting the 4 circles went on and on withot stopping.
I can come in the Recovery, Power off, Start and Restart bootloader modes but everything stayes dead....
Hope there is any help or support.
Robert
Hi Robert!
I do not have my Nexus 7 at hand (I'm traveling),... so maybe the following is helpful:
I did find a good post at the XDA forum which shows the basic and a good list of additional guides.
The topic is also being discussed at StackOverflow, Nexus ADB not visible under Windows, and Android StackExchange.
I know it's a lot of info, and I wish I could be more helpful ... 
