Hi Zhiheng, Chimera keeps the map in memory if it is being displayed. You can see if your maps are in memory using the Chimera volume dialog menu entry Features / Data Display Options and pressing the data cache "Current use" button. If your map is displayed with step size > 1 then only a subsampled copy of the map is in memory (e.g. every other data plane). The fact that you get an IO error when trying to save the map from Chimera with volume dialog menu entry File / Save map as... suggests that the map is not all in memory and is trying to read the map in order to write the full resolution file. If you sent the Chimera traceback I could give a more definitive answer. There is no capability to write out just the subsampled part of the map which is in memory. Tom