The concept of VDI is to store and run desktop workloads including a Windows client operating system, applications, and data in a server-based virtual machine (VM) in a data center;
And as I see in your screenshot, you are running VirtualBox. So you are simply installing Ubuntu in a virtual environment and in that environment only your 40GB Virtual drive is recognizable to Ubuntu.
It's like you only have a hard drive with the capacity of 40GB. Actually VirtualBox will virtualize every piece of hardware for you. So everything including CPU, Memory, Hard Disk, etc are being managed by VirtualBox. Read more about Virtualization.
If you want to share stuff between your Ubuntu and Windows, you should install
virtualbox guest additions in your Ubuntu guest; Then share a directory using VirtualBox configurations.