Basically, I put CyanogenMod on it last year, and then didn't like it and ended up putting a hacked-up copy of the stock ROM on it, which didn't work properly so a couple of weeks ago I put a testing release of CyanogenMod 11 on it, and so far I'm happy, except for a couple of small things which are a little irritating. Immediately after being wiped and reflashed, it reported about half the storage as being used, and when I look at the filesystem from my main computer, it's saying the same - about 5GiB on the device, about 2GiB used... and about 600MiB available. So somewhere, there's a lump of *something* taking up a huge chunk of my storage.
I'm prepared to do a full wipe, however full I can, and reflash it - provided I can keep the recovery environment (Philz Touch). But how can I do this? I've looked through all the options available in the OS and in the recovery environment, and found nothing I haven't already tried.