I check, 500GB free on the target, 20GB file. I try making a directory.... works fine. I try copying a small file.... same error.
After checking logs and whatnot I realize that this is probably a machine that hasn't been on in a very long time. Check the clock... It's 1999, fix it. Error goes away.
I'm sorry, Apple... how can a clock set to before the turn of the century make it believe that there isn't enough disk space. Do the greater-than/less-than operators change meaning depending on the season?