exporting a virtualbox VDI to a harddrive to boot it natively

You can export a VDI to a raw disk image with the following command:

VBoxManage clonemedium --format RAW debian.vdi debian.img

You can then use the dd command or any suitable disk imaging software to write the image to the disk.

Since you are using Windows as your host system, you can proceed in 2 steps.

1. Convert your VDI into a raw IMG with the command:

   VBoxManage.exe clonehd --format RAW source.vdi destination.img
   

2. Then use a tool like HDDRawCopy to copy the raw IMG to your HDD.