If all of the cables are tight on both the logic board end and the where the drive sleds plug in from what I've been reading it's a bad logic board, they call it the backplane board too.

It seems like a common problem.

You can get an internal SATA card for around $30, Rocket620A. Macmall has them so they are OS X compatible, but OWC has them cheaper. I don't know if the wiring you have can be re-routed or if you can use the cables that come with it to attach to the back of the sled holder. Supposedly this one works in a Mac Pro. Not sure if it's bootable tough.