Carbon Copy Cloner 4 introduces a brand-new interface and support for Yosemite. The upgrade begs the question, though, why can't we "fix" CCC 3.5 so it will run on Yosemite? It's not nearly as simple as you might think...
In the last several years, Apple has demonstrated its intention to push out a new operating system on an annual basis. Concurrent to those new OSes, users see updates to nearly every application they use -- updates that accommodate changes in the new OS. Some updates are small -- minor bug fixes, while others are large, e.g. to accommodate major new OS features. Carbon Copy Cloner is no different, each OS brings not only user interface changes, but also changes to various filesystem and networking features that directly affect how CCC can perform backup tasks.
Carbon Copy Cloner 3.0 was developed between 2007 and 2009, so it was based on OS X system facilities that were available on Tiger. Between then and now, we've updated CCC 3.x to add support for new OS features, such as HFS filesystem compression, Notification Center, the Recovery HD partition, full disk encryption, and new sleep/wake mechanisms. But there are architectural limitations to the choices that we made back in 2007 that can't simply be updated piecemeal. While we've worked really hard to make Tiger-age technologies work for us on newer OSes, these most recent OS releases have painted a very clear picture -- it was time to retire the old architecture. That decision was made just in time, because our worst fears were realized in Yosemite's earliest developer previews -- CCC 3.5 was badly broken on Yosemite, and we were facing many, many months of work to get it working reliably.
At that time (shortly after the Yosemite announcement), we were at a fork in the road. We were nearly finished with a new generation of CCC -- just months away from releasing it. Do we put that work on hiatus for another year and invest precious development resources on bandaid solutions, only to face an unendurable support avalanche when Yosemite arrives? Or do we tear off the bandaid and invest all of our resources into the next major release of CCC, which was nearly ready and already working on Yosemite? The decision was painful, but it wasn't difficult. Investing in 7-year old technology just isn't a winning strategy, not on Apple's platform, not with Apple's "pace of innovation".
CCC 4 is a huge upgrade, it isn't anything like the OS compatibility updates that we've provided for free for the last 7 major OS releases. For folks that have a CCC 3 license, we want to be as fair as possible for the upgrade. We're offering a generous "lookback" period -- anyone that purchased a CCC 3 license since Apple announced Yosemite on June 2, 2014 will get a free CCC 4 license. Anyone that purchased a personal use license prior to that can get the upgrade at 50% off. That's just $20, and the license covers every Mac in your home. More details on upgrade eligibility are available here.