Either A or B will work, but on an MD3000 you'll have to add the additional disks to the diskgroup in the GUI 1 disk at a time. Option A will take hours to days, depending on the disk sizes, disk spindle speeds and production IO. After growing the diskgroup you can either extend the virtual disk(s) or create new virtual disks. The option to grow a virtual disk is command line only (smcli (part of the storage manager software install)). Option B will be available nearly immediately (but the virtual disk will be scrubbing, so it'll not be full performance potential till it's done), but you will need to move some of your data to the second disk (do not use dynamic disk to merge the 2 disks into a single drive letter if you're using Windows). Performance wise a raid 5 is best off limited to 6 or 7 drives. With 7200 rpm drives, once you get to 6 or 7 drives and cannot afford to make a separate/2nd raid 5, I'd recommend raid 6 instead of raid 5.
Scenario: MD3000 with one disk group / 4 virtual disks assigned to a W2K8 Server with Cluster Services. Available disk space is critically low. Have a RAID 5+1 hot spare in bays 0-5.  No drive in bay 6. Have no drives in 7-14.  Have 4 spare drives to use to create new space. Have 2 HBA Host Ports, one assigned to each server in the cluster.   Questions: Would it be better to: A.  Add Free Capacity to the current disk group to be assigned to virtual disks? or B. Create a new RAID Array, plus a new disk group and assign the disk group to the cluster?   Unknowns: 1.  Is the risk of data loss high with either of these scenarios? 2.  I assume my configuration would not allow expanding the drive capacity in Disk Management if I was to add the free capacity to the current disk group.  In other words, the space would show up as an additional drive instead of capacity at the end of an existing partition.  I would be forced to create an additional partition to use the space.  I'm looking for a 2nd opinion on this.  Also what is the risk to data loss in this scenario? 3.  Can I create a new RAID Array, New Disk Group, and New Virtual Disks (I'm plan on just creating 1 of each), and assign it to the cluster?  Is there risk of data loss in this scenario? Which of the above scenario's is the least risky, and or is the best practice?  Is there another way, that I have not considered? Knowns: I do have good back ups, and I can plan for the worst case scenario.  We also have failover facilities if I need to use them.  We have already backed up our databases and imported them into the back up system.  However if the potential for data loss is high, it will affect my scheduling.
MD3000 W2K8 CLUSTER ADDING STORAGE