Hi,
Well, there is a reason behind it, when the Manual and carton says, that you should not connect 2 unit's at once
Were you using same DVDRW Drive all the time?
I assume that you overloaded the BUS or the unit itself with 2 devices. It also happens that Windows is really the trouble maker. S-ATA worked on my testing machine all the time, but suddenly w/o any particular change of software or hardware, it stopped working on that machine. On a different machine it works fine. So my recommendation is to try at least 3 different PCs.
Cheers,
Shinigami