I'm not exactly sure what you're asking due to the terminology, but if you're asking how to remove a harddrive from a disk group, there's no way to do this. You'll have to back up your data, delete the virtual disks, then delete the disk group, and recreate the disk group (with fewer drives) and recreate the virtual disks (and restore data).

The term 'assigning' sounds like you may be talking about what the MD3000i calls 'mapping'; to assign a virtual disk to a server. Servers can be put in a host group (only to be used when clustering (e.g. Microsoft Cluster services, VMware HA/DRS cluster, etc)), which makes me think this latter might also be what you're asking about.

Sorry about the confusion. Let me be more specific. If I delete a Vritual Disk. Does that space become available to the SAN again for use or does it stay as free space only within the Disk Group?

I want to get rid of one virtual disk and use that space to create a sperate disk group.

By deleting the virtual disk you only make space in the disk group.

To get some drives to make a different disk group, you'll need to put more drives in the system (or daisy chain an MD1000 (max is 2 MD1000's) to get more drive slots), or destroy an existing disk group (destroying all data on the other virtual disks in that diskgroup) and recreating it smaller.

Basically; you can expand with data in place, but to shrink anything (virtual disk and/or disk group) you're looking at a; back-up, destroy, recreate smaller, and restore data.

For clarification, when a Disk Group is created then physical disk drives are allocated to a RAID group.   From that point onward when Virtual Disks (VD) are created within the Disk Group (DG) a portion of the space within the Disk Group is allocated to the VD from the overall free space in the DG.   Later deletion of the VD will deallocate the space, returning it to free space within the Disk Group.  

Even if the capacity used for a Virtual Disk is (coincidentally) equal to that for a physical disk, there is no correlation, as that capacity is striped across multiple disks in the RAID group.  When that VD is deleted, the striped space becomes available as free space in the Disk Group.  It does not deallocate a physical disk from the RAID configuration.