This is a very common occurrence in Dell computers. I have personally installed a 2nd drive as the primary slave device, jumpered correctly, and when booted the system, got a "No OS found" error. Well I called Dell just to make sure I wasn't losing my mind. I had a senior technician tell me to go into the bios, and on the main screen make sure that the Num Lock, Scroll Lock, and Cap Lock lights were all on, then press Alt-E, Alt-F, and Esc/F10 to exit and save changes. What this strange key combinations does for you is flush the hardware configuration that the Dell has stored somewhere and makes it look at everything all over again. The first time you boot will be a lot slower but it will boot just fine that time and every other time after that. Well at least until you change your hardware config again, then you will have to do the above all over again. Hope this helps.