RESTORE FROM ONLINE BACKUP

As an Exchange Administrator you are always prepre for disaster, for that purpose i setup
a lab environment and install a Exchange 5.5 sp4
with a Arcserve backup agent for Exchange installed exactly similar to the production server
(same organization,site name).I tried to restore
from a online backup tape few times so far it hasn’t been(error(s) -1811 jeterrfilenot found )
successful.Any clue in this problems? Thanks GaryT