automount

We are accessing data from our server via nfs. The nfs exports are automounted on the nfs clients' computers. It works fine. However there is a partition (a whole drive in fact) on our server that is automounted on demand. We wanted this hard drive to unmount automatically if it is not needed to save energy and reduce noise levels. When I include the mountpoint of this partition in /etc/exports everything seems to work fine, but automounting this export on a client machine fails. It can still be mounted manually though with a regular mount command.