Disk space not freed up after deleting files and emptying Trash

I had approximately 50Gb of Windows VMs on my MacBook Pro internal SSD (128Gb) (OS X 10.7.3). I copied them to an external drive, then deleted them from the internal drive, then emptied the Trash. When I look at disk usage in Activity Monitor it shows the same level of usage as it did prior to deleting 50Gb of files. Any ideas why the space hasn't been freed up?