Hello Wee Jin,   No, you can't just add in an off the shelf hard drive.  The EQL array requires drives from Dell only.  They have custom firmware on them.   Also you have to match the RPM speed as well. You can't mix speeds in the same member.   Regards,   Don 
Hi all, We recieved a hand me down PS4000X configured using Raid 10 and out of warranty. 1 of the 450GB SAS HDD has failed. Dell support is waiting manufacturer reply if they still can provide this particular HDD modelSeagate Cheetah st3450802ss . I understand the PS4000X supports 400/450/600GB SAS HDD. But its an uncommon capacity and hard to find new replacement. Would it be possible if i just replace the faulty SAS HDD with a 500GB or 1TB SAS HDD? If possible will there be any issue that may arise? 
EqualLogic PS4000X HDD Replacement