Hello,   You are running older EQL firmware.  It created a new RAIDset using the full 600GB drive size instead of 300GB.  So if a 600GB drive should fail the 300GB drive won't be able to replace it.   In the short term you will need to get another 600GB drive and replace the 300GB spare.   That will stop the message.  All future drives as they fail must also be 600GB.    Regards,  Don   
We've faced the issue while we extend HDD by insert full 24 slots.  - Previously 12 HDD are 300GB of capacity - Insert new 12 HDD Capacity size 600GB  We have 2 spare HDD 300GB and 600GB but spare 300GB alert : Disk too small. How to fix this issue while customer don't see the alert ?  Thanks for advance.    
EqualLogic PS6100 (Disk too small)