Just some advice - I changed the map file like described on DC Rainmaker and left the original file in the trash of my mac (this was yesterday). I used the device and got the device full message. When I plugged it back in, I was surprised to see there was very little space on my 520. I noticed my old map file showed up in the trash (backup is elsewhere). When I erased - it went back to 65.1mb of storage.
I think this applies to my 920xt as well. On Mac OS, unless you empty trash, the file remains on device even if it is unseen.