As expected (by those wiser than me), most of the sources that are detected in GALEX but not SDSS are small technical problems with either catalog. In many cases they are GALEX artifacts. In others they are SDSS dropped fields or dropped sources (usually because they are too close to the edge of the survey boundary). A lot are GALEX sources in the faces of big, nearby galaxies, which GALEX software splits up into sub-sources differently than SDSS (see an example below, in an enormous nearby galaxy—can you tell which one?). So I wrote code today to catch all of these cases. It is running now. I hope we find some true GALEX-only sources.