Thanks for your help. As I said, I cannot attach the drive to IDE1 for purely mechanical reasons. Had already tried MS/SL/CS: no difference.
I'm not really any wiser, but it now seems OK. I updated the m/b BIOS firmware and that didn't help (rather the opposite, as it is now behaving oddly!). I then took out the Samsung burner and put back my Pioneer one and it hooked onto DMA automatically. I tried the Samsung one in a different computer and it also DMAed correctly. I then took out the Pioneer and put back the Samsung and it would not enable DMA. In desperation, I took it out again and put in a CD-ROM and it enabled DMA. This satisfied me that the computer was OK. I re-installed the Samsung and, this time, it did have the DMA enabled. Bloody computers!