It’s the PERC in the R710. RHEL 6.2 or so and up doesn’t like a boot disk that’s more than 2 TB. I have an R710 with 6x1TB disks in it slated to be a search node for Security Onion (CentOS 7 based) and once I made the first 1 TB and the other five TBs separate, it boots CentOS just fine. With all six disks in one array, it went to the GRUB menu after a CentOS 7 install as you described. The Security Onion install disc would just poop its pants immediately after booting. I found this out after much head scratching and googling. Once I get a sufficient supply of round tuits I will get the 5x1TB RAID partitioned as SO wants it to be and get SO installed. But it gets worse - RHEL 8 and up don’t have support for the PERC so a separate driver disk is needed at install time for RHEL 8 and derivatives.