My mini started acting slowly recently. Then one day it hung on the grey boot up screen. So I replaced what seemed to be a faulty HDD (a WD 1TB Scorpio Blue 5400 rpm) with a new one (a WD 1TB Travelstar 7200 rpm). The mini would not recognize the new drive when mounted internally. So I put it in an enclosure and formatted it with my MBP. Here is where things get odd... I connected the new HDD to the mini by USB, and disk utilities recognized it. I took it out of the enclosure and again mounted it internally and DU would not recognize it. Well, must be a logic board of cable problem. Right? Well... Then I took another HDD I have the contains a CCC clone of my MBP, and connected it to the mini via USB, and the Mini booted from it. Then I removed it from the enclosure and mounted it internally. Again the mini booted just fine -- which should be impossible if the problem is the logic board or any connectors. So... 1. new HDD is not recognized internally, but is recognized over USB. 2. MBP CCC cloned HDD is recognized internally, and over USB. In other words the problem can't be the new HDD because it works fine over USB. But it the problem also can't be the logic board or connectors because the Mini boots fine from another HDD. Can anyone shed any light on what the real problem might be, and how I can complete my HDD replacement task?? I'm completely stumped.