If a VirtualBox virtual machine can’t see Shared Folders in your Windows guest with Linux host, fix by:

In Linux Terminal:

adduser $(whoami) vboxusers

In Virtualbox Windows guest, click Install Guest Additions in the VirtualBox menu Run setup on the virtual drive that appears. Reboot the Windows guest. From Windows guest Command Prompt:

net use x: \\vboxsvr\share

where “share” is the name assigned in the VirtualBox shared folders dialog.