FDR & CVR mounts
I assume that one of the reasons (other than listening capabilities) that France has dispatched a sub is to use radar/sonar to map the floor to look for large pieces of wreckage.
There is some hope that the aft section of the fuselage may remain in fairly large chunks so, rather than just searching for the FDR & CDRs, they are looking for the much larger pieces of the aft fuselage.
I would hope the US Navy would be aiding with their undersea maps of this area to help the search for anomalies.