Several things may impact disk performance. Some general suggestions:
1. Make sure you have latest firmware/driver installed. They're available here: Download Center
2. Always choose hard drives from the Server hard drive validation test report
3. Avoid chassis vibration
4. Check your HDDs for bad sectors
Thank you for the suggestions. I had thought I had updated all the drivers and firmware. I looked at them all and discovered that the 631xESB/6321ESB driver was out of date, 8.x
I updated it to 9.1.1, rebooted, and now my Crystal Disk Mark read / write speeds are in the mid 90's. Similar to the speeds I receive with the HP ML350 g6
Thank you for your assistance.