Problem in wiping cache partition?
try this if you haven't figured it out yet..

https://productforums.google.com/forum/#!topic/nexus/HRALDucgBq4

Wiping the System Cache Partition
Wiping the system cache partition will not delete any files or data but always remember to backup anything important on a regular basis, ie/ photographs, music or document files.

If your device is switched on, power it off.

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You'll see an image of an Android robot with a red exclamation mark

While holding the Power button, tap and release the Volume Up button - a menu should appear. (Do not hold the Volume Up button down as this will reboot the device).
Use the volume buttons to scroll to "wipe cache partition" and press the Power button to select it.