Retrospect 6.1 is not Snow Leopard compatible.  Dantz has confirmed this in their Knowledgbase article 9723.  Backing up a Snow Leopard Mac using Retrospect 6.1 may damage the system.  Apparently, just doing a backup of the boot drive may result in expanding all HFS compress files.  This causes the system to take up an additional 4-5GB of space, and you’ll see hundreds of thousands of “different modification date/time” errors in the Retrospect log.  Although the Mac will run in this state, the only way to fix this damage is to reinstall Snow Leopard.

At the moment, Dantz only supports backing up Macs running Snow Leopard with Retrospect 8.1.622.  They have additionally stated “An update to EMC Retrospect 7 for Windows will soon be available to support Snow Leopard.”  Retrospect 6 will not be updated..