First run the virtual box and on the windows installation click settings. And from storage option disconnected the drive. Then go the drive location and run the following command to increase the disc size into 20 Giga byte.
vboxmanage modifyhd win1064.vdi --resize 20000
Add from settings add disc. Choose adding existing one, and add the one you removed. Run the virtual windows and go to disc manager on windows. From there click on the system partition select extend partition. Your will be able to extend it up to the space you’ve added there.