Few years ago, I asked same question. My Sansa c240 v2 does not work with OF and microSD card when RB is installed.
I mean, is that the original software will not turn on if the SD card is inserted. Stuck on the "Sandisk" logo.
If I take out the card, original firmware starts without a problem.
Six years have passed, hundreds of updates, and the problem is still not resolved :-(