Volumes with no assigned letter are set with Automount=false during the installation of Storage Foundation for Windows 5.1 SP1
Setting AutoMount to False is a deliberate action taken during the installation of Storage Foundation 5.1 SP1. This was a deliberate design decision taken to mitigate issues seen in Clustering, Backup and Snapshotting caused by automatic drive letter alignment. This is not to be considered as an issue is Storage Foundation 5.1, but as a precautionary setting.
Setting AutoMount=true using MS Diskpart utility after the istallation od SFW 5.1 SP1
Windows 2008 R2