Thanks - HDTune did indeed help me solve the problem. It reported a warning related to a SMART factor that could indicate a bad cable. I checked the MB and the cable in question was indeed loose. After resetting it, no more atapi problems! Its a shame that neither the Seagate nor WD tools reported that issue and showed the SMART status always as green...
Which one SMART factor could indicate a bad cable?? I never heard about this.