-
The UPURS support uses the NMI area in page &D to load data. Loading into that region won't cause a crash, as it might with disks, but the results will probably be nonsense.
-
The serial connection can be a bit sensitive to programs accessing the user VIA. The filing system will try to rescue itself if it looks like things have got into trouble - this process should be transparent, and hopefully all you'll notice is a slight delay.
One example is Exile, which writes to the user VIA as part of its sideways RAM detection.