I got an R4-Revolution (its the same like an M3 DS Simply).
It works with my x1.7 mod and an Kingston 1 GB microSD card (japanese). I didnt test the Sandisk yet.
My M3DS Real doesn't work with x1.7 and Kingston or Sandisk.
So it seems also to be a matter of the backup device.
I tested some original cardridges with x1.7, nearly all seems to support the x1.7
ChildrenOfMana: works
SuperMario DS: works (its much nicer

)
Castlevania: works
Another code: works, but no speedup
Bomberman: problems when saving
AdvanceWars DualStrike: works