These instructions assure you are using Windows as your host and Ubuntu as the client machine. I am using vbox 4 and Ubuntu 10.04 LTS. I had loads of issues getting this to work with other tutorials and forum posts. I kept getting errors like:

/sbin/mount.vboxsf: mounting failed with the error: Protocol error
/sbin/mount.vboxsf: mounting failed with error: No such device

Here is my method which worked for me and hopefully for you as well.

Stage 1. Install Guest Additions. If you have done this already skip this stage.

  • Start Ubuntu and click devices in the vbox window, the select install Guest Additions
  • This should put a cd icon on the Unbuntu desktop, double clicking should open a folder.
  • Click “Open Autorun Prompt” then wait for it to install.
  • Shut down Ubuntu from within the client.

Stage 2. Add the shared folder

  • Open up VirtualBox control panel.
  • Right hand click on the box you want to share a folder with
  • Go to share folder, add a folder and tick Auto-Mount and Make Permanent
  • Click OK and start your Ubuntu vbox

Stage 3. Give Your User Permission

  • You need to add your user to the vboxsf group.
  • Click system->administration->Users and Groups
  • Click Manage Group -> scroll down, scroll down and click vboxsf. Then click properties.
  • There should be a list of Group Memebers, make sure your user is clicked (by default your user will be called Ubuntu).
  • Close everything and restart from within Ubuntu, not vbox control panel.

Stage 4. Done

  • Once Ubuntu starts your shared folder can be found in “File System/Media/”