I have a situation here with my Nexus 4. I am running CWM Recovery 184.108.40.206 and AOKP nightly builds. I was using Stock ROM before AOKP. That time, before I upgraded I was trying to make a nandroid backup. I had around 7GB free space before performing the backup. Eventually the backup went wrong, it said "Error while backing up /data". I don't know what to do so I just reboot and re-run the backup process, but it still gave me the same error. I then boot into my phone and realized something... my free memory space has lessened to around 2GB only! I then decided not to make the backup and just flash my ROM to AOKP. I thought this will clear the problem, but it's not.
So, my question is, how to clear this used space? I checked in the nandroid backup folder and found absolutely nothing there, so it must be somewhere else... Any help would be appreciated! Thanks in advance