you can try to copy raster and set the unknown crs by tool from toolbox. Than try to set georeference to new file

you can try to copy raster and set the unknown crs by tool from toolbox. Than try to set georeference to new file

EDIT technicly you can do this by this tool (from your screenshoot I figured, that you have arcgis 10.x) so:

1. copy raster by arctoolbox->data Managment->Raster->Raster Dataset->copy raster
2. in ArcCatalog find new copied raster and that right mouse button on it->properties->Spatial reference->Edit->Clear

so no you have tiff with no georeferencing. Try now to georectify raster.