Hello, copyback is an option to enable automatic rebuild or not, when there is a failed disk. By Default is set to On, I suggest you to leave activated.   Integrated Dell Remote Access Controller 9 Attribute Registry | Dell US     Disk cache policy, When this feature is enabled, the physical disk writes data to the physical disk cache before writing it tothe physical disk. Because it is faster to write data to the cache than to a disk, enabling this feature canimprove system performance.
Cant find a good explanation for what Copyback mode is on the PERC H730.   Same for "Disk Cache Policy" - I understand the writeback/through modes but why have a enable/disable disk cache policy? 
PERC H730 Copyback mode