Re: RAID level for SATA 1TB on CX4-120
For backup with SATA drives, using RAID 6 would be prudent.
I note you have not specified many TB of storage you require. I'll assume your backup is a small percentage of whats available. I recommend you create 3x (8+2) RAID groups. The (8+2) is regularly used for backup-type RAID groups with backup. These RAID groups exceed your bandwidth requirement. Assign one drive as a hot spare. Use the other four drives as scratch drives.
Splitting across DAEs on a CX4-120 has no effect. It only has a single bus.
Sizing of RAID groups, Bus Balancing, and SATA Drive Characteristics, as well as recommendations on RAID group provisioning can be found in the EMC CLARiiON Best Practices for Performance and Availability. In particular, the 'Storage System Sizing and Planning' section at the end is recommended reading.