So after install a clean install of El Capitan, I installed a new 6TB RED hard drive in a Thunderbolt 2 bay to back my my 6TB media drive holding 3TB. Blackmagic Disk test rates each drive at 170mb per second read and write. When I try to do an initial back of Time Machine, its fast for the first 75gb, then slows to a crawl changing from minutes to days. In the last 7 hours its only backed up 15gb. Has anyone seen something similar or have any ideas? This never happened in Yosemite.