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.
Unfortunately, Windows only knows FAT or NTFS.
Originally all my data was in HFS+ journaled case sensitive. When Mavericks could not read these, I had to change all my drives to HFS+ journaled not case sensitive. When Apple refused to honor their warranty, I had to convert every drive to NTFS. When the larger drives came out, I had to use Ext4.
Instead of constantly changing file systems, I believe that a better solution is just to fix the Ubuntu NTFS driver.
Robert Pearson
Hello rpearsonii
Regarding better NTFS drivers in Linux:
https://www.theregister.com/2021/09/06/github_merges_useless_garbage_says/
There has been some "discussion" around this new driver being included in the Kernal. 
Yes! I have seen Linus Thorvald's comments about the Paragon Software drivers. I actually purchased the Paragon Software driver for HFS+ on Ubuntu. I never got it to work. I talked with their tech. They said, "it works fine on our virtual system". Never got it to work on mine. A friend let me use his Mac to copy volumes to NTFS volumes. The driver is expected to work in kernel 15.5. Latest Ubuntu is 15.4.
Maybe, some year…
Robert Pearson