Shadow Location (Using CREATE SHADOW)

From InterBase

On non-NFS systems, which includes all Microsoft Windows machines, the shadow must reside on the same host as the database. You cannot specify a different host name or a mapped drive as the location of the shadow.

On UNIX systems, it is possible to place the shadow on any NFS-mounted directory, but you run the risk of losing the shadow if you experience problems with NFS, so this is not a recommended procedure.