Software updates always corrupt
I've been told this sounds like a dying hard drive, so I got a brand new drive and installed Snow Leopard fresh onto it. Now, Software Update doesn't even suggest 10.6.3, and attempts to download the combo updater still result in corrupted DMGs.
I'm really not sure what the problem could be, at this point. Is there a weird hardware quirk in the Mac Pros (I purchased mine in January of 2007) that could explain this? Or anything else I could try?
Mac Pro (early 2007), Mac OS X (10.6.2)