Have you tried using a Raster Mosaic Dataset?
Do you have any control over whether these users can be brought up to a more current version of ArcGIS than 9.3?
Thanks for the reply....It worked with 5 images...then I loaded about another 50 and left it overnight. In the morning I was greeted with the same old error! and 64bit Processing set-up? what´s that...and how do I do it?
No...they are external groups.