Greetings,
What exactly do you mean .adf raster? It's probably an Esri GRID without looking at it. GRID's are made up of many .adf files in folder structure like that of a coverage. Esri TINs are also made up of multiple .adf files. Do you have an Info folder associated with this raster? A screenshot of the workspace where the raster resides would be helpful. If you can see your data (.adf) in ArcMap you should be able to just run Copy Raster and create a Tiff file.
Regards,
Eric