This week I revived a 2007. 2,1 Dual Quad 3Ghz that a Japanese friend managed to cripple. 1. The CPU B ( lowerCPU ) 3 Ghz X5365 is definitely dead so I have removed it + heatsink & sealed the motherboard socket with plastic until a replacement arrives from AliExpress. 2. Installed two spinner HDDs HDD 1. El Capitan 10.11.6 - Everything basically working OK except Facetime. I haven't checked every app yet. HDD 2. Windows 7 64 bit - Everything working OK. 3. Installed an EFI flashed HIS HD 5870 1Gb. Working. 4. This is the shocker . . . the amount of DUST I found under the CPUs made me cringe involuntarily. ( For CPU novitiates, removing 2,1 CPUs & heatsinks is pretty easy as the CPU(s) are inserted to the socket and correctly tortioned by the CPU CLAMP. ) I advise everyone owning one of these old cMPs to think about devoting an afternoon to de-dust it thoroughly. Including the cheesegrater holes too ! - I'm serious. This is what I found under the CPUs. Both motherboard sockets were like this. CPU A ( top if cMP is vertical )) CPU B ( lower ) It could well be that CPU B got "cooked" ! Very hard, deteriorated thermal paste - took ages to remove, almost like solder. This photo was taken before final cleaning to mint condition. Conclusion : The hardest part of this operation was removing the REAR FAN but now I can remove & re-install both CPUs & heatsinks in about 10 minutes - practice makes perfect. I gently cleaned ( with isopropyl alcohol + soft haired calligraphy brush ) and re-thermal pasted both CPUs both the dead CPU B X5365 and the motherboard CPU B motherboard socket. = No joy. If, after the re-placement 3 Ghz X5365 arrives that BOTH CPUs do not boot up then I will leave this 2,1 cMP with just the TOP CPU A installed, after all a single 3 Ghz X5365 is nothing to sneeze at and this 2,1 in it's current ( cleaned ) state is more than adequate for my Japanese friend's needs. Next I'm going to do some serious research as to what are the most causes for CPUs to fail. As it stands, this 2,1 is running very well now in both El Capitan ( updated from Mavericks 10.9.5 ) despite only having one CPU running.