The error "Could not open raster dataset from file" occurs when peforming certain functions using the Raster Calculator.
In ArcGIS 8.1, non-grid outputs from the Raster Calculator are not added directly to the Table of Contents in ArcMap. Examples of non-grid output include shapefiles (such as that generated from GridShape) and tables.
The non-grid output is stored in your working directory. As a workaround, navigate to the location of these outputs and add them to ArcMap using the Add Data button on the Standard toolbar.
Some output types cannot be added directly to ArcMap, such as the result from the GridAscii function.