A VDI file is a VirtualBox Disk Image, a proprietary format used exclusively by the Oracle VM VirtualBox software. Using a VDI, you can create a complete image (or "clone") of your hard drive, including settings and programs. You can then boot from the VDI on the same computer. The process to make a VDI file itself is simple, but because it is backing up an entire hard drive, it could take up to several hours.
Open Oracle VM VirtualBox.
Click the "New" button.
Click the hard drive you want to back up as a VDI file.
Select "VirtualBox Disk Image (*.vdi)" next to the "Format" drop-down box.
Type in a name for the VDI and click "Save." A progress bar will appear, showing you how long until your VDI is complete.