text: For array type MDisks, the system automatically sets its tier because it knows the capabilities 
  of array members, physical drives, or modules. External MDisks needs manual tier 
  assignment when they are added to a storage pool. 
  
  **Note:**The tier of MDisks mapped from certain types of IBM System Storage Enterprise 
  Flash is fixed to tier0_flash, and cannot be changed. 
  
  Although IBM Storwize V7000 can distinguish four tiers, Easy Tier manages only a three tier 
  storage architecture. MDisk tiers are mapped to Easy Tier tiers, depending on the pool 
  configuration. 
  
  Figure 10-2 shows the possible combinations for the pool configuration with four MDisk tiers. 