[I will wait a bit, to see if anyone comes up with a real solution for mirroring the whole user folder without having to go through all of the trouble this solution requires. I really do want to accept somebody else's answer!]
After some more research, I have now come to believe that you cannot mirror the whole user folder, but you can effectively do the same thing by changing the location of each of the special subfolders to be on the mirrored drive.
What I show from here on is only for the Documents folder. Here are the steps:
- Create a Mirrored Drive (I used M for "Mirrored" drive).
- Go to the Documents folder, and right-click => Properties
- Select the Location Tab
- Choose Move and put the location of the Documents folder on the Mirrored drive.
My Documents folder was moved from
C:\Users\JoeUser\Documents
to
M:\Users\JoeUser\Documents
Then point everyone who expects it to be on C: to go to M: with a symbolic link:
(Using a command window (cmd.exe) run As Administrator...)
MkLink.exe /D C:\Users\JoeUser\Documents M:\Users\JoeUser\Documents
Creating a shortcut in any of the standard ways should also work.
- Any hardcoded paths starting with the original Documents folder will be broken and will have to be changed. If you set up the machine this way to start with, then this step should not have to be done, but each setup will either automatically find the Documents folder or be pointed there as part of the setup.
What follows is a more detailed set of steps with screenshots for those who want more detail:
- Free up some space on two different physical disks to create a mirrored disk.
A. in Disk Management, Had to right-click-"Shrink Volume..." on Disk 1
to match the 48 GB of unallocated space on Disk 2.
- Create the Mirrored drive
A. in Disk Management, right-click on the Unallocated 48.29 GB of disk 1, choose "New Mirrored Volume..." and click Next through the intro screen

Select "Disk 2" and click "Add>"
Click "Next>"

On next screen, choose "Assign the following drive letter:" option, and figure out what drive letter you want to use. I used "M" which for me stands for "Mirrored Drive". Click "Next>" button.

I used the NTFS file system, chose the default allocation size, made the label "Mirrored" and checked "Perform a quick format", then clicked "Next>" and then the "Finish" button to complete setting up the mirrored drive.

The mirrored drive clearly shows in Disk Management (maroon color).

Find the Documents folder and right-click-"Properties".
Then select the "Location" tab.

Change the "C:" C-drive designation to "M:" m-drive (the mirror).
Click "OK"

Click "Yes" to create the full new path.

Click "Yes" to move all the documents files over to the mirror.

Wait for files to copy from the C: drive to the Mirror M: drive. If your documents folder is big, this could take a while.

And now for an important detail, the symbolic link (or shortcut) from C: to M: leaving others a pointer to where the files really are:
In a command window having Administrator rights, run this command:
MkLink /D C:\Users\JoeUser\Documents M:\Users\JoeUser\Documents
