I have somehow run out of space on my LG G3. Under storage under Misc I see have a 6.34GB System Data file.
I tried dialling
*#9900# as suggested here in order to delete the dumpstate/logcat but when I dial
*#9900# it tells me it's an invalid MMI code. http://androidforums.com/threads/low-on-space-system-data-huge.278837/
Note that my phone is not rooted. I can get to the hidden diagnostic menu using the international code shown here, i.e.
3845#*855#, however I can't see any menu items that look like they might clear that system file.