Whenever a backup is run, Time Machine says "Preparing Backup...". During this stage, it looks for what files have been changed since the last backup by comparing changes in the Event Log (I assume) with its cache of files on the system.
Because of the cache, the Preparing stage is usually quite fast (30 seconds), but for some reason, sometimes when I restart from Boot Camp (Windows 7) into OS X, Time Machine's cache is wiped clean or corrupted. I know this because the Preparing stage takes 10 minutes after booting back into OS X.
So, that's why I'm wondering where the cache is located so perhaps I could determine why it's sometimes corrupted.