Alex Chaffee's blog
First of all, fie on Apple for giving both their cloud storage service and their backup program names that are almost completely google-proof. They've recently corrected one of those by renaming "dot mac" to "MobileMe" but calling your backup program "Backup" is a great way to make it really hard to investigate. It's like, imagine how hard it would be to do a background check on someone named John Doe.
So I use the Dot Mac Backup and it works pretty smoothly, which is the second most important feature in a backup program. (The most important feature is the ability to actually restore files.) But then one day it said that to incrementally back up my "Home Minus Media" set -- the set containing my Home Folder, but excluding big-ticket items like Music, Movies, Backups, Downloads, and so on -- would require 63 DVDs. WTF?
It turned out that the problem occurred after I trashed a few old DVD rips that I had finished watching, and the culprit was the directory /Users/chaffee/.Trash. Seems like the UI was helpfully excluding it from the list of subdirectories of /Users/chaffee, it being a system file and all, so I couldn't mark it to exclude. That's OK, I think, I'm a power user, so I'll just check the box that says "Show invisible system files."
Except there's no such box. Try as I might, I can't find a way to exclude the Trash folder from the UI. I had to dig into the file system and edit Backup's own data file, as follows.