First, I've heard others report that the NEC 25x0's don't like being slave, though I haven't tested this theory myself, and have also heard others say that've run theirs as slave successfully.
Second, reguarding that LED lighting up when the drive is working, this is normal for the NECs.
Third, though your bios reports the controller being in DMA mode, make sure your bios is set as well. I have an Asus P4R800-V Deluxe and found the bios reporting the channel as in DMA mode while the bios was actualy set improperly for the drive. If you have a section for SB100 EIDE controller or something of that sort, look there. If not, just go over each page in the bios with a fine-toothed comb.
Best of luck.