Problem scenario: You are trying to mount a file share using Oracle VirtualBox. You have a Windows host and a Linux CentOS guest. (For complete directions on how to set up a folder share for this scenario, see this link.)
You are trying to run this command: "mount -t vboxsf contint /mnt/windows-share"
# where "contint" is the name of the folder share in the Oracle VirtualBox GUI.
But you get this error:
"/sbin/mount.vboxsf: mounting failed with the error: Protocol error"
How do you solve this apparent "Protocol" problem?
Verify the name of the folder share by going to the GUI for the guest VM. Then go to Devices -> Shared Folders -> Shared Folder Settings. In the window that pops up, see the "Name" column on the left.
Replace "contint" in the command with the name of the folder share as seen in the "Name" column. The command should now work.