[Brian]

Enginpost, this is most commonly when the disk image file is incomplete, either because it's still downloading, or because the connection dropped before the download had actually completed.

In cases like that, the fix is just to pull the file down again and give it plenty of time to complete and/or compare the size of the downloaded file in the Finder with the size posted on the website.

This isn't the only cause - there are other things that can cause the disk image to fail to mount, but they're much rarer and require some more investigation to diagnose/fix. If you call our support ninjas at 800.315.6664 or email us, we'll be happy to assist!