The error message occurs because smartctl is trying to talk directly to a virtual RAID volume (e.g., /dev/sda ) instead of the physical hard drives hidden behind the controller.
Fix: smartctl "Open Device /dev/sda Failed" on Dell MegaRAID Controllers "smartctl open device: /dev/sda failed: DELL or MegaRAID
This error occurs because the operating system sees your RAID controller (e.g., a LSI MegaRAID please try adding '-d megaraid