Hikvision Hvr4.0 Download

I have a hikvision NVR DS-7604NI-E1/4P and i am trying to export video files to a flash drive attached to the USB port. I'm using the IVMS-4200 software to search for the file i want to backup via the remote configuration menu. I can select the usb disk from the drop down box but it gives an error saying that there is no space on the usb drive, however the disk is empty.