You’re a normal user, and you use Time Machine on your Mac. You’ve got a portable USB on your Airport, or maybe one of the fancy Time Capsules with the hard drive inside. And you think to yourself, “Why is my hard drive filling up?”
The main reason is that Time Machine puts local backups on your hard drive until it’s 80% full. The more you use your Mac, the faster it fills up. It’s not you; it’s just versions of files that are kept and you’re not aware generally that it isn’t moving over to your backup drive. Well, it is, but it’s also being kept on that otherwise-spacious new driver.
Here’s how you turn it off in Terminal, and still use Time Machine normally:
sudo tmutil disablelocal
You’ll be asked for your password. This simply ends the local storage of backups that you’re already storing on your external drive.