I believe that FF/RW is not possible on m2ts files over a certain length on the x700. People ran into that problem with the Lord of the Rings extended cuts. If you search this thread, you'll find some posts about the issue.
The only workaround is to use TSmuxer to create a Blu-Ray folder output using the mpls of the main film and place those folders created by TSmuxer in the root of the USB drive. Then you'll be able to FF/RW. The catch is that you can only have one film in the root at a time.