In thread in my country , I saw tut for unlock boot loader without lose data in first time In nexus 10 I think in nexus 7 2013 work too but I did not test it yet.
the method is
1. Put recovery image to sdcard.
Save to sdcard
3. Boot to bootloader and use command
fastboot boot recovery.img
4. Install zip file for root
6. Install BootUnlocker and Open
You can unlockboot loader without lose data
#### I did not it yet ####
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