Try connecting one of the Netgear's switch ports to one of the Sagem's switch ports. No further configuration required, although switching off the WAP and DHCP service on the Netgear would be a good idea.
Just don't use the "internet" uplink port on the Netgear - unless you actually want to create a separate IP network.
I've done that with a Netgear hanging off a Linksys. Having said that, I was just using the network ports, not a USB connected HDD.
SD