I'm guessing it's not safe to use "newIORef" as the function argument to withStorableArray, but in a simple case [1], it seems to work okay. I suppose the danger is if the RTS decides to move the array. Does this happen IRL?

The danger is rather that the array might get freed. A StorableArray contains a ForeignPtr to the array, and the finalizer on the ForeignPtr will free the array after the last use. If you obtain a pointer to the array by nefarious means (ie. 'withStorableArray return' would do it) then the RTS might free the array while you still have a pointer to it. Cheers, Simon