when I am trying to merge some raster files into one raster file the function fails and error 999999 is reported
What type of rasters? Esri grids? Are you using the Mosaic to New Raster tool? Did you specify the analysis extents in the tool you are using to be the union of inputs? Are there any spaces in the folder where you are saving the result or are you trying to save it to a geodatabase....etc...any details will help