I am having the exact same problem as mentioned in this original post.
I have placed my second SATA HDD (400 GB Wester Digital) into every other available SATA port and I get them same result. It starts the PC, recognises all HDDs in the BIOS, shows the vista loading logo and then goes blank and just humms there forever.
Only solution is to unplug the second SATA HDD and it loads fine.
I have also tried a different power cable to the drive as well as a new SATA cable, all with the same result.