Exclude Files from Time Machine
By default, Time Machine backs up all the files on your hard disk. That's usually exactly what you want. However, in a few situations you may want to exclude certain items from being backed up.
One obvious reason is a lack of space: if you have too much data on your hard disk to fit comfortably on your backup disk, something has to go. Out of concern for privacy or security, you may wish to leave certain sensitive files out of your backups. Another big reason is performance: some files are so large and change so frequently that they keep Time Machine busy doing virtually continuous backups. Read Items to Consider Excluding, next page, for advice about what Time Machine should not back up.
To make sure an item doesn't ...