Folders can be mounted or unmounted manually or automatically for both Windows and Linux.
For Microsoft Windows go to Windows Explorer and look for the shared folder by navigating to My Networking Places | Entire Network | VirtualBox Shared Folders. Right-click on the shared folder and select Map network drive from the pop-up screen; you can assign a drive letter to that shared folder.
Starting from VirtualBox 4.0 this folder can be automounted, to automount a folder in Windows the shared folder will receive its own individual drive letter depending on the free drive letters remaining in the guest.
In the Linux guest, use the following command:
mount -t vboxsf [-o OPTIONS] sharename mountpoint
To mount a shared folder automatically ...