[Nexus 7 Q&A, Help & Troubleshooting] alscott51:
Sideload fix

Managed to fix this without losing anything (sdcard, docs, pictures, apps, setting etc). Took a while mostly due to lack of understanding of fastboot and ADB sideload from the recovery options.

Lots of posts advising to boot into the bootloader and use adb and fastboot. Everything I read to do with using fastboot commands via the nexus bootloader (insatalling boot/system/recovery etc) involved having an unlocked bootloader or having to unlock it as in my situation. This of course involves a factory reset of the Nexus resulting in a loss of the data that I was trying to retrieve, so i ruled this option out.

As suggested I should try the ADB pull command, but because I had not previosly enabled the USB debugging option via the setting menu everything I read said this was not an option. Read in one forum post something like "As you have not enabled USB debugging ADB is dead to you, just flash a factory image via recovery".... and lose all your data... this is not what I wanted to hear so I abandoned this option aswell.

Then I discovered sideloading. With this you can manually install an OTA update (the ones that pop up in your notification window saying something like "System update available") via the Nexus recovery option "apply update from adb". This does not invlove wiping any data , apps etc as it would just install the differences between what you are currently running eg. 4.4.2 and the updated android you want to go to eg 4.4.3. This is different to flashing a full factory image via fastboot were everything would be erased/formatted and an entirely new install would be flashed. Of course... simple when you know what the option is for and what you are doing. Of course I had had no clue what sideloading was all about.

So with my new knowledge I downloaded an OTA update from here:

http://xdaforums.com/showthread.php?t=1745781&highlight=4+23

but wasnt sure what version of KitKat was running prior to the bootloop. I knew it was somewhere around 4.4.2.. so took a chance and tried KTU48L from KOT49H (4.4.3 from 4.4.2) and copied it to the Minimal ADB folder.

I booted the Nexus to the recovery mode and selected "apply update from adb". The tablet then conected to the PC (windows) as shown by Device manager. (I used Koush's universal ADB driver as windows failed to find anything of use). https://plus.google.com/103583939320326217147/posts/BQ5iYJEaaEH

I opened Minimal ADB downloaded from here

http://xdaforums.com/showthread.php?t=2317790

and typed "adb devices" which showed the Nexus was connected.

I then typed "adb sideload nexus OTA file name.zip"

Lots of sending files messages appeared in the adb window the Nexus disconnected from the PC and showed this...

Finding update package...

Opening update package...

Verifying update package...

Insatlling update...

Verifying current system...

Removing unneeded files...

Patching system files...

Patching boot image....

Unpacking new files...

Unpacking new recovery...

Symlinks and permissions..

Patching remaining system files...

script succeeded: result was [/system]

Install fromADB complete.

All looked good so I rebooted the Nexus but I was still stuck in a bootloop at the four spinning circles. I tried clearing the cache from the recovery menu several times powering it down and leaving it to boot but after half an hour accepted failure.

So I tried downloading KTU84P from KTU84L (4.4.4 from 4.4.3) and repeated the procedure. this time not as much files seemed to be sent to the Nexus, I guess this was just a small update. Again it showed "script succeeded" but still failed to get past the spinning circles of the boot animation.

Thinking I was defeated again I was going to give up but tried downloading LRX21P from KTU84P (5.0 from 4.4.4). This time when I sideloaded a mass of files were sent to the Nexus much much more then the other two updates, obviously KitKat 4.4.4 to lollipop 5.0 was a big update. Again I got the "script succeeded" but this time I got the new Lollipop boot animation and the Nexus fully booted.

I can only guess that the big update to Lollipop removed or replaced files or did something to replace the faultly part of my install that other OTA updates did not change.

Anyway I did not find any other fix that advised sideloading an OTA update to fix a bootloop, maybe I just looked in the wrong places. I tried to write this in laymans terms for non techie people in the hope that it may help someone else in the same situation who like me knows very little about android.

Thanks to the forum for the information...

Also link here with some info about half way down page on effects of usb debugging being enabled for sideloading, and near the bottom of the page a good link to a sideload pdf guide.

https://productforums.google.com/forum/#!topic/nexus/Fhl6VVb0kio