I was about to copy a ~35GB file onto my Mac. I got the “can’t be copied because there isn’t enough free space” message. I checked Finder and the available space was ~75GB.
First reaction: WTF?
Tried a few more times with no success, much like when you keep on switching on/off a broken switch. Then I checked “System information” and I saw the real available space was ~28GB. Same reaction: WTF?
After some digging I discovered that it’s Time Machine’s fault. Besides keeping your backup on the designated (usually external) drive, it also keeps a copy on the local drive! So here goes the third: WTF?
Fortunately you can disable it via a simple
sudo tmutil disablelocal
This will ask for the admin password and disable the local copy. In case it does not free space immediately just start the Time Machine backup.
To enable it again (do you really want to?)
sudo tmutil enablelocal
ps: All this happened on Mavericks 10.9.5. I am not sure how Yosemite behaves.