Are you sure it didn't say 'No New Navigation Data Found?'
I think the unit compares what's in the internal memory to what's on the SD card and will put any new data into memory.
As long as you just exported your Navigation data, it just copied what's in the system memory to the SD card. It will retain what was in the system memory.
If you deleted your Navigation data at any point, then it wiped out what was in the system memory.