I am trying to run the Slope tool on a DEM mosaic I have. The original DEMs came from USGS and are in NAD UTM 1983. I have mosaiced them into a single dataset. I repeatedly get the dreaded Error 999999 when I run the Slope tool, but no messages indicating why. It fails as soon as it starts, even though my computer usually takes several moments to run a tool. Can anyone think of anything else I might be doing wrong here to cause the error?