You normally set the IP address on the network drive itself and not the router. So if you can log into your network drive's browser interface and it offers static IP addresses as an option, set it there.
Just pick an address that is one of the valid addreses for your subnet e.g. 10.0.0.250 if you have a 10.0.0.x local network when you configure the network drive. If you also need the gateway address it is the one for your network so for a 10.0.0.x network it is 10.0.0.1