dCache installations, used as a frontend to tertiary storage system, need, at some point, to exchange data this such a system in order to store new, precious files and to retrieve files from the HSM if not yet, or no longer, available on one of the dCache pools. Unfortunately there is no well defined interface for such HSM operations. So the dCache overcomes this problem by calling configurable (dCache external) shell scripts or binaries whenever an HSM store or retrieve operation becomes necessary. The local HSM administrator is responsible for providing this procedure and to make it available and known to the dCache. This small writeup defines the way dCache will call such an external method.