If I interpret your question correctly, you want to put each subfolder of the moodledata directory on its own lun? If that is the case, it seems like a lot of effort without a lot benefit. If you want to manage disk growth more effectively you will want to explore things like volume groups and using filesystems that are able to be grown hot. I'm speaking primarily with regards to linux but in theory its the same method for windows & unix servers. This way as you need more space, you add a lun to the volume group and expand. Depending on your storage backend configuration, you could benefit in performance as well if done correctly.