First be sure that you are accessing a dataset below the top level of the pool, rather than the pool directly. You should be able to drag and drop files between any two folders on that dataset but if you are dragging between datasets, they are treated like two separate file systems and the files will be copied, rather than moved. Does that answer your question?

Try adding both users to a Group and assigning permission to read and/or write to that dataset to the Group.

We suggest that you create a dataset below the top level of the pool and copy your files to it. This will give you full permissions control and not conflict with the system dataset or Plugin datasets. You may need to copy and delete the files in steps if you pool does not have the available space to copy everything at once. Doing so will also allow you to snapshot and replicate that dataset independently of the others.

Have you tried upgrading to TrueNAS CORE?

No, you don’t need to add both. You are correct, all users with the group specified in the auxiliary group list will receive those permissions.