Years ago I purchased an HD 4870x2. The thing stopped working just after the warranty expired, so I purchased a 560 ti which has been running fine. Last week I decided to upgrade my mobo/ram/cpu and just for the sake of curiosity, I decided to pop in that old 4870 and it worked! I have all my old parts in another case ( different power supply, but it's 750w so it should be fine ) and the 4870 won't work in that machine. The same machine that ran it for the first year and was running a 560 ti until a week ago just won't run that card and I'd like to figure out why.
tl;dr: 4870x2 works in one machine, but not in the other. How can I figure out why?