My 2tb drive external drive is perfectly fine; it’s also new. The problem is with Mojave. Disk Warrior indicates the backup partitions are in use, so that is why DU cannot access them. This situation came up with the internal drive when I was downloading some files. DU gave the same message. I terminated the download, then DU was able to access the partitions.
The question is: what is using the backup partitions? Then, how to stop this ‘improper’ USE. I think the problem may lie with Superduper backup cloning.