appleTV: sharing aperture library on external disk

I moved my Aperture library from the iMac to an external hard disk in order to free memory capacity on the iMac. Adjusted the settings in iTunes in order to instruct AppleTV to use the Aperture library on the external HD. It fails completely. Checked the permissions of the library on the HD and they are R/W, so that cannot be the issue. Anyone familiar with what I have missed?