When you have finished
partitioning your drive(s) and you have made a full-system drive image, it's time to start moving things around. I stumbled across this method which became so immediately obvious that it was a face-palm moment. You have your full-system drive image. At this point you should have your original Windows 10 installation in its original partition, and two empty, formatted 100GB partitions. First, in your new Users partition, create a Users folder. Mount your drive image. I use Image For Windows, and it mounts images files as Z:\.
With your drive image mounted, you can copy what you want, where you want it. I found a couple of tools that make this and the other necessary operations quite painless. First, download Process Hacker Portable. It will need a couple of plugins from Process Hacker Forums, TrustedInstallerPlugin_x32 and TrustedInstallerPlugin_x64, downloadable separately as .zip files. Instructions for their use is right there at the download site. Using Process Hacker in a portable wrapper means that it isn't necessary to install it in Windows, it will run just fine from its own folder.
Some of the files in the Users folder are protected, and Process Hacker run as Trusted Installer greatly simplifies the task. Open Process Hacker, then in the upper left corner click Hacker, and in the dropdown select Run as trusted installer. A command box will open. Type (without the quotes) "cmd.exe" and click OK. That will open an elevated Command Prompt. You'll be running robocopy (I'm using my drive letters for demonstration).
The command line for robocopy should be
robocopy z:users\ v:\users /v /s /e /copyall /im /xjd /xd "System Volume Information" /xd "$RECYCLEBIN" /r:1 /w:1 /mt:64 /log:v:\users\yourusername\desktop\users-copy.log"
and then hit Enter. Depending on the number of Users and number of files in each User's subfolders, this might take a bit. Once it finishes successfully, unmount your drive image and make a fresh drive image of your new Users partition.
This brings us to the next tool we will need to use. There are very many registry entries that must be changed to point to the new Users location, and that would get extremely tedious. Funduc Software has Registry Toolkit available as Shareware in 32bit and 64bit versions. The shareware version is limited to 50 uses, per the EULA: "If you are using the Shareware version you are hereby licensed to use this software for evaluation purposes without charge for a period of 50 uses. If you use this software after the 50 use evaluation period a registration fee is due." The retail version is $25, and I bought a license a long time ago (I use it quite often in my piddlin', such as figuring out how to do this).
The A side of my dual-boot has only two users, and the registry contains 206 values and 1,109 data items for "V:\Users" that were changed from "C:\Users" in only a couple of minutes using Registry Toolkit. Registry Toolkit on its own (Run as Administrator) cannot change values or data items owned by Trusted Installer. However, if you first open Resource Hacker, select Run as trusted installer, then enter the path for the Registry Toolkit executable (for me it's U:\Program Files\RegTkt\RT64.exe) and click OK, it's a walk in the park. Once those registry edits are complete, reboot.
After the reboot, rename your C:\User folder to something like C:\UsersOld, and reboot again. If the second reboot is successful, use your PC normally for a bit, open and close programs (AppData is in the Users folder), and satisfy yourself that Windows is operating normally. If you notice any glitches, you can open regedit.exe under Resource Hacker Run as trusted installer, look for any rogue C:\Users leftovers, edit any that you find to point to your new Users partition, then reboot and check again. Satisfy yourself that everything is alright before you proceed to the next step.