Just loaded XBMC not realising my NAS wasnt on, all my libraries got wiped because I had added the following entry in my advanced data config:

<advancedsettings>   
   <videolibrary>
<cleanonupdate>true</cleanonupdate> 
   
  </videolibrary>
</advancedsettings>
Dammit. Total rescan needed now, and it never gets all the tags right! Gutted.