@Brian K OK, I just turned it off, and that message didn't appear, this time. Also, I deselected "Virtual Drives". But, I am not sure about these settings for the backup. These must be default, because I didn't select them. Should I amend anything at this stage?
https://www.minitool.com have a couple of disk management tools you may want to test. Their partition management is much more powerful than most people ever need.
CMD in Administrator mode: Microsoft Windows [Version 10.0.19043.1826] (c) Microsoft Corporation. All rights reserved. C:\windows\system32>DISM /Online /Set-ReservedStorageState /State:disabled Deployment Image Servicing and Management tool Version: 10.0.19041.844 Image Version: 10.0.19043.1826 The operation completed successfully. C:\windows\system32>