I used my own working SE/30 to test the hard drive. It's definately not the hard drive or the formatter.

I tried multiple drives with Apple HD SC 7.3.5 and also FWB 2.0.6: both work fine in my good SE/30 when doing exactly the same, while they don't with the "problem" board.

Also the working HDD with os 7.5.5 from my good SE/30 does not boot at all with the problematic board. So I just swapped the logic board in my good SE/30 but no boot at all. It's polling the HDD but that's all.