VirtualBox/SharedFolders

• Select Devices -> Shared Folders

Choose the 'Add' button.

On the Windows Guest, run
net use x: \\vboxsvr\share

On the Linux Guest, run
sudo mount -t vboxsf -o uid=$UID,gid=$(id -g) share ~/host

https://help.ubuntu.com/community/VirtualBox/SharedFolders