FFXIAH.com

I'm not keen on the inner workings of how an Xbox360 functions, however I would assume that the xbox in essence registers itself with a disc drive and any outside source will lock it up preventing it's use. Possibly an attempt to avoid people from modding it to death like the old xbox was prone to. Since you already opened em up and voided the warranty I would re-replace the disc drives and see if it clicks back into place to read your discs again.