Problem formatting Nuvi 2350 and 2455 units

I have now two messed up Nuvi units a 2350 and 2455, both act the same, was typical filesystem error stuff, backed up the filesystem then formatted using RMPrep with the correct settings, now all is well but the firmware does not see the newly created dos filesystem, I can see it when using GarminCure but the firmware does not see it. I am thinking I will need to use the fftl command but unsure of the use or if this is the right thing to do, the only mention I have found was this usage; fftl,0,1:/2.bin

Any help would be great.

Chris