I used a fresh set of host and an fresh installed ununtu 12.04 with an fresh virtualbox to share a directory of the host to the vboxinstance, when I ran into the problem that i got always an error when I tried to write (copy a file) to the host: Error: "Protocol error" Probably most of you see my mistake in a second.... it took me some time to find it myself. Reason: Ever tried to create a file with an ":" in windows? It is not allowed and exactly that is told by the errormessage. Solution: Dont try to copy something with a filename which is not allowed by the hostfilesystem. Rename it to something allowed. Doh!