I have been unable to get my SIDE1 (which I no longer have - thanks SJ) and also my SIDE2 to load ATRs. I get a "No PBI driver found" error. I am also unable to use MATR via SDX - when I try to mount an image, it just says "Error".
I have an Ultimate 1MB built by candle, and I have tried updating the ROM several times (using Jay's ROM builder). I have tried both the 0.3.4 PBI BIOS from flashjazcat's website, as well as 0.3.5 (not sure where I got that). The U1MB setup screen shows the SIDE and SIDE PBI config options. However, when I hold down the L key, all I get are repeating key clicks.
I am not sure what to try next.