There are a few explanations for this occurring. You might try emptying your Recycle Bin on your Desktop. If that doesn’t work, depending on how your system is setup, you may have "Windows System Recovery" option turned on that is still occupying the space on the drive. That space is used by Windows to store file system information.

The only way to truly delete the contents of the drive is to regenerate the encryption key and reformat the drive.

In order to regenerate the DL3’s encryption key, power on the device, touch Setup > System, then go to the second page and touch Zeroize Drive. Be aware that Zeroizing the drive initiates the destruction of all encryption keys and user passwords, making the data on the drive irretrievable. The DL3 will be reset to the original factory state. The DL3 will need to be reinitialized and formatted in order for it to be redeployed.

Reformatting the drive can be done using the DataLocker Disk Formatter tool which can be accessed by going to http://datalocker.com/techsupport/downloads/.