Why Time Machine takes forever to ''prepare backup'' ?

If you choose to connect your backup drive only from time-to-time, try to do so at least every-other day, at minimum. Otherwise, it may take several hours of computation just to decide what needs to be backed up, before any files are transferred.

Time Machine uses a System-wide list of recent changes to Folders known as the File System Event Store to decide what needs to be backed up. If you wait too long, the File System Event Store is stale, and cannot be relied upon for an accurate and complete list of changes. Then Time machine must perform a deep traversal, inspecting every file on your drive and comparing to the modified date on every file on your backup. That takes all afternoon before anything is copied.

The late James Pond wrote a series of accessible, illustrated articles on many aspects of Time Machine. This article (continued on the pages following by using the [Next] link at the bottom of each page) covers these issues:

.Pondini's Time machine tips-- How Time Machine Works its Magic

Carbon Copy Cloner does the job within about half an hour at most!

Carbon Copy Cloner will slow your machine to a complete STOP while it does its copying.

Time machine's "claim to fame" is that it is the backup that gets done, because it does not ruin performance of the rest of the computer while doing its backup operations. You do not have to set aside a "Special Time" when you only do backups. When you need it, a Time Machine Backup is much more likely to be there (and be current).

You can continue to do your regular work while it does this. The first Full Backup is by far the biggest backup. After that, it will work quietly and automatically in the background, without interrupting your regular work, and only save the incremental changes.