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I spent a number of hours trying to get a shared folder set up between a virtual machine running Raspbian and my host machine (Windows 7) but it just wasn't working. Finally, after pulling together instructions from several different web sites and trying the things they all seemed to agree on, it works. I made myself these notes in case I have to do it again later; I thought they might be of help to someone else as well. Please be careful to note which steps are performed on the *host* and which on the *guest* OS. That's part of where I was going wrong originally.
sudo usermod -G vboxsf -a <your username on the virtual OS> (adds user to virtual box shared files group)
log out (of the virtual OS) log in (on the virtual OS)
Shared folder(s) should appear in /media folder.