Are your Kickstart ROM sockets in good order? The pins of the FastATA are much thicker than the ROM chips, and your ROM chips may not be making good contact now after many insertion and removals of the FastATA (which could weaken the sprung contacts).
I know when I first had my MkIII all I got was a black screen - no where in the documentation did it tell me that Kickstart 3.1 was required to function, as I was running 3.0 at the time; I tried a 3.1 Kickstart and it worked. I never once removed the FastATA, because I realised at the time it wouldn't be such a great thing to do. I never had to use the included tie-wrap as it's locked in there like cement... I don't think I dare ever take it off.
