Yeah, I've checked it, formatted it, etc.
It works in every other device. It seems like it's just this Sansa Clip+ that's causing problems.
And I posted on the rockbox forums, but it happens with the regular firmware aswell, except with that it just hangs on Updating Database or whatever it is.
It seems similar to
this guy's thread and he only fixed it by replacing the Sansa Clip.
I'm going to try replacing the card and seeing how that works out.