I have found out from Apple that problems downloading Lion and Mountain Lion from the App Store can be because of the type of router BT supply, which limits file size to 2GB. As Mountain Lion is 4.4GB, it will start to download and then stop, and say error, or calculating, or just disappear!!! Ring BT to check your router on 0800 111 4567. The old black and silver, mesh sided 2WIRE router will be replaced for a new model which allows large downloads, on a next day delivery!!! Hope this helps at least one person avoid the stress I have had!!