text: However, you can adjust this configuration by setting the number of drives of different classes 
  to zero. For information about Easy Tier, see Chapter 10, “Advanced features for storage 
  efficiency” on page 403. 
  
  If you are adding storage to a pool with storage already assigned, the existing storage is 
  taken into consideration. Some properties are inherited from existing arrays for a specific 
  drive class. Systems aim to achieve an optimally balanced configuration, so it is not possible 
  to add significantly different MDisks to one pool with the given GUI dialog. 
  
  For example, if the pool has an array MDisk made of 16 drives of DRAID6, you cannot add 
  two drives of RAID1 to the same pool. Otherwise, an imbalanced storage pool is created. 
  
  You can still add any array of any configuration to an existing pool by using the CLI. 
  
  When you are satisfied with the configuration presented, click**Assign**. The RAID arrays, or 
  MDisks, are then created and initialized in the background. You can monitor the progress of 
  the initialization process by selecting the corresponding task under**Running Tasks**in the 
  upper-right corner of GUI screen, as shown in Figure 6-39. The array is available for I/O 
  during this process. 