I just ran into this issue moving around large VHD images. I found the cause to be local time machines backups. Time machine seems to keep backups on your local (i.e boot) drive by default, in addition to your nominated Time Machine drive. This is meant to be transparent and only consume free space, but clearly it's not smart enough to free the space when you actually need it.
If you launch the "System Information" utility, on the "Storage" tab you should see a large portion of your primary hard-disk consumed by "Backups".
To disable local time machine backups, run the folllowing at the terminal:
sudo tmutil disablelocal
This should immediately free up the space. If not, try running a Time Machine backup manually to kick it into gear. You may re-enable local Time Machine backups with the following:
sudo tmutil enablelocal
Hope this helps.
Tom