VirtualBox

if you are playing with
http://www.virtualbox.org/

and trying to share a folder between guest os and host os

mount -t vboxsf
example
mount -t vboxsf installdir mountdir

if you bump into this error
/sbin/mount.vboxsf: mounting failed with the error: Protocol error

first thing you need to check is ,if sharename matches the value in the name field of the shared folder in the virtualbox ui

ok you dont understand see the picture and look for the red oval,

so the correct one is
mount -t vboxsf installer mountdir