I experienced that problem with the VT6421, but recently somebody said VIA had been solved it, and I recently confirmed that with several brands of drives configured for 3.0Gbit. The VT6421A I tried was dated 4th week of 2010 (1004CD). But HDAT2 wouldn't run with that controller and hanged during the drive detection phase. OTOH Windows XP Home ran very sluggishly with the primary hard drive plugged into an Intel G41 based motherboard and the secondary drive into a Silicon Image SiL3512. This didn't happen with VT6421A and Promise 20378 cards.
