I created a test cluster in Windows 2003 running on Virtual Box. The virtual disk to be used as a cluster resource has been changed to type shereable using VBoxManage command line utility. On that disk I created a primary partition and mounted it on one server as drive D. The cluster has been created successfully using Majority Node Set for quorum.
When adding drive D as a resource it failes and becomes unavailible in explorer, removing the disk from the resources makes it availible again.
To eliminate the Shereable vdi disk as the cause I created a new virtual disk on a new virtual SCSI controller, which does not have Shereable for it's Type. Still when added as a cluster resource it fails and becomes unavailible.
Last I checked, VirtualBox virtual disks are not supported for Cluster Services. You should use a third VM to present this disk as iSCSI rather than as a shared vdisk.