The storage radar in LinuxMCE does not know about /dev/vdX devices, and would need to be amended to use it.
The relevant code is in StorageDevices_Radar.sh and StorageDevices_StatusRadar.sh in /usr/pluto/bin (or in src/PlutoStorageDevices if you've checked out the source tree.), with a little digging you should see what to add.
Failing that, you could always just add the device manually to the device tree using Advanced > Configuration > Devices in the web admin, selecting your CORE (as I am assuming this is a disk attached to your core), and adding a child device of device template #1790, and then filling in the appropriate blanks,
PK_users - either public, a specific user, or use LinuxMCE directory structure (since this is an existing disk which hasn't been used with LMCE before I am assuming, this option isn't relevant to you...but you should look into LinuxMCE directory Structure for any new blank disks going forward. It provides for distinction between public and private media, as well as nice convienient places to put your media.)
Filesystem - the filesystem used as reported by the kernel
Online - This needs to be checked so the system knows the disk is available
Capacity - human readable capacity value (shown in Manage Drives)
However, I would strongly ask that you amend the storage radar, so that this benefits not only you, but other users of this type of setup in the long run.
Of course, if this is too much to begin with, you could just..oh..I don't know...USE A STANDARD STORAGE MECHANISM PERHAPS?
(seriously, all you sysadmin types are going to fucking drive me to drinking. We realistically can't support all your zillions of edge cases for system environments that you guys want to willy nilly slap together without first understanding the system! I grow weary from it, and am THIS close to really spewing some nasty words about it.)