This was a bug. It's fixed in ArcGIS 10.1. You can convert the rst file into tif or img file, then open it in ArcMap 10. To convert, go to File->Import->GDALIDRISI, in tab gdal_translate, input is the rst file, provide output, and for output format, choose GTiff or HFA. After done, you can open the output file in ArcMap 10. Are there any options aside from using arcgis 10.1? 😞
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