OK, I fixed mine without losing any data by following the instructions below, copied from
here.
EDIT: To clarify, I have a 2012 wifi so used JWR66Y factory image and then used ADB sideload to install the kitkat zip update which I found on this page.
link
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I was in the same situation and ran into that issue as well. What I did was download the 4.3 factory image from here:
https://developers.google.com/androi...s/images?hl=vi
Make sure you get the correct one for your device. I have a 2013 wifi so I chose: Android 4.3 (JSS15R)
Inside that downloaded file, you'll find image-razor-jss15r.zip. It's a couple of levels down. Unzip the files from there to your SDK folder (should be boot.img, recovery.img, system.img, userdata.img, and cache.img).
Boot your Nexus 7 to the bootloader (volume down + power) and connect to your PC with USB cable. Do NOT go into Recovery mode yet.
From the command prompt, make sure you can see your device: fastboot devices
Then, run the following commands:
fastboot flash boot boot.img
fastboot flash recovery recovery.img
fastboot erase system
fastboot flash system system.img
At this point you have a clean install of 4.3 JSS15R.
Now go to Recovery mode and sideload the 4.4 OTA as you originally tried and it should work this time.
I followed these steps and was able to update to 4.4 without losing any data. YMMV