I purchased a game download from Amazon for my new iMac - Call of Duty 4. I am getting the following error when I try to install: The following disk images couldn't be opened CallOfDuty4.dmg - invalid checksum I have deleted everything and re-downloaded and tried to install this game several times. Each time it stops midway through the download and gives me an error message, but once I double click on the installer, it resumes and finishes the download, but once I try to install, I get the message above. Any way to fix this problem? I tried calling Amazon's help line, but they really couldn't help. They have offered a refund, but I got a great deal on this game - $9.99. I would rather try to get this one to work.