How to create a shared folder between the host operating system and the guest operating system.

Mac OS X Host

First, create the folder to be shared on the host OS, for example, /Users/dw/virtualbox-share.

Debian Guest

1. Start Debian in VirtualBox
2. VirtualBox menu: Devices > Shared Folder Settings…
3. Add the folder created on Mac, select Make permanent
4. Create a folder on Debian, e.g. /home/dw/Desktop/host
5. Append the following to /etc/fstab on Debian (get the values for gid and uid from the id command):

    debian  /home/dw/Desktop/host vboxsf umask=0022,gid=1000,uid=1000
   

6. Restart Debian

Windows Guest

1. Start Windows in VirtualBox
2. VirtualBox menu: Devices > Shared Folder Settings…
3. Add the folder created on Mac, select Auto mount, select Make permanent
4. In the Windows Explorer, go to Network > VBOXSVR > \VBOXSVR\windows
5. Right-click on \VBOXSVR\windows and select Map network drive…
6. Choose a drive letter and add a shortcut on the desktop