If you are using a 512 Mbyte RPi you must have a firmware later than about October 2012 (I don't know the exact date.) The larger RAM chip on the 512 versions is not supported (= won't work) with earlier firmwares. I don't know the age of hexxeh r3 but the filename of raspbian_wheezy_20120608.img.7z which is downloadable from http://www.linuxsystems.it/2012/06/rasp ... mal-image/
suggests it was put together in June 2012 which was months before a firmware for 512 Mbyte RPis was in circulation.
Update. I looked at the page given above for hexxeh r3 and noticed:
Changes from the previous r2 image are as follows:
Updated to the latest Raspberry Pi firmware and kernel as of 13th May
So that is much too old also.