answer:CarbonCopyCloner is free. SuperDuper is $27.95 for full functionality. I would tend to assume that any slowness perceived would be a function of how much data was in the incremental change, as well as what type of connection is used for the external drive. For example, Apple’s USB 2.0 implementation is noticeably slower than Windows… hence Firewire’s popularity on the Mac platform. So, you need to take more into account than just the software. Under the covers, many of those are very similar in how they operate.