Ubuntu MATE works fine with a small number (< 8) of small (< 6 TB) external USB hard drives. When I plug in a 6TB drive linux goes into ultra slow mode (slower than glacial - more like continental drift). Copying 31.563GB takes 66 hours as opposed to 3TB in less than 48 hours with the drive unmounted. Thinking it might be the drive, I exchanged it for an 8 TB drive. Same problem. Reported it as an Ubuntu-bug with no response. Modern 3.5" hard drives are as large as 12 TB, so this problem needs to be addressed. My best guess is that someone writing Ubuntu chose an algorithm that works well with 4TB drives but definitely does not for 6TB or larger drives (problem of scale).
Try to use another file system.