GPS overwrites tracks before 100% full?

i have a garmin etrex hcx gps and it keeps overwriting my tracks. you would think it would fill the memory up until 100% full but the last couple tracks i have made it starts overwriting at about 70-80%. i have unclicked the wrap when full button but it still is overwriting before its 100% full. its really frustrating casue i dont know when it will start deleting my tracks and sometimes i need the track to find my way back on a hike. would it have something to do with the memory? i have a 1gb card in it right now but it can hold 2. any help is appreciated.