You would need to check with your VAR or Sales rep.  I don't recall ever seeing a SAS to SATA disk upgrade kit ever being offered for the PS4000.  It might actually be  more cost effective to get a PS4100E than trying to upgrade the SAS array.  Since you can have up to 24 drives in the PS4100.   Regards, 
Hi All,We've recently been told by one of our consultants that our EqualLogic PS4000X cannot be upgraded from the existing 600GB disks to anything larger. The reason given for this was the particular model has a hardware limitation which restricts it to 600GB or less drives.If true, this is really frustrating since it's barely 2 years old and cost almost £20k! I'm wondering if anyone can confirm or deny this?Thanks in advance.Gareth
EqualLogic PS4000X Disk Upgrade