For 'norebuild' option with sderaster -o mosaic command it should explain that the 'norebuild' option is used to prevent the raster dataset from having to rebuild pyramids on the entire raster dataset. This could occur if you were batch mosaicking and one of the images had a different resampling method than the existing raster dataset had or one of the images required that a new origin point (pyramid reference point) had to be determined. Specifying the norebuild option would prevent the pyramids being dropped and recreated.