We are having the same problem.
Mac mini server, OS 10.9.5.
In our case, drives have been successfully mounted for months. We keep our server on 24/7, but turn off computers that access it every day. Suddenly, today, no access to one of the raid drives on the server (even as other raid drives were accessible). After much fussing, repairing drives with disk utility, restarting server, the drives were all accessible from other drives on the LAN, but still not visible on the Mac mini desktop itself. Disk Utility says they are mounted, but I cannot see them locally, only on the network.