You will need a couple of tools...

You must have the ability to work on your Windows installation from the outside, as well as when booted into Windows. You can either copy the contents of your Windows 10 installation USB/DVD to a small partition on your hard drive, or you can use Windows Help and Support to create a system repair disk, and copy that to your hard drive (it launches and operates from the hard drive much more quickly than from the Optical drive). The repair disk is bootable, as is the Windows USB/DVD, but my preference is to copy the contents to a dedicated small partition and use EasyBCD from NeoSmart Technologies to make the partition bootable. The repair disk is considerably smaller than the Windows DVD. Having either one installed in its own small bootable partition may well come in handy down the road. You will also need a ~100GB empty partition down the road, large enough to contain the mounted image of your Windows installation, and label it Mount. I'll explain later.

If your imaging software has the ability to mount an image file while booted from its rescue disk, you won't need that ~100GB partition. Terabyte Image For Windows TBWinRE boot USB can do that, but I'm unfamiliar with any other imaging software.

The Command Prompt in Windows Repair has System and Administrator level privileges. Use it with caution, as it is quite powerful. You will also be using Diskpart. If you aren't familiar with Diskpart, you need to familiarize yourself; open it, learn the commands and switches, how and where to use them. It's best to get that under your belt before you get started with these procedures.

You can set up your system however you like. I'll tell you how I have mine, and you can take it from there. I have the Windows OS isolated in its own primary partition in one SSD, which is C:. I have the Program Files folder in a logical drive in a primary partition, drive U:, on my second SSD. The Users folder is in another primary partition, drive V:, on my third SSD. Each of these partitions is 100GB in size. I use drive letters far down the alphabet to prevent any future additions of partitions or devices from changing my drive lettering scheme, which would cause difficulties. The Recovery Disk/Windows DVD is on still another logical drive, and I have a catchall logical drive on the OS hard drive for downloads and extraneous unclassified other files. I have a few other logical drives for particular file types, such as music, pictures, and such.

All these machinations are performed using a lot of registry editing (there are tools to help). This procedure is not extremely difficult, but there are a number of steps.

Based on some of the feedback I have received, please note: It is important that all the steps are followed sequentially as specified in these instructions. Doing things out of order or skipping a step may well take you someplace you don't really want to go. You don't have to move all the folders mentioned in these procedures; you can skip moving a particular folder. But when moving a folder, you must follow the procedure for moving that folder step-by-step in the order listed, all the way through. Also, adding steps or including items not mentioned in these pages may well cause issues from which you may have extreme difficulty recovering.

I can help with problems encountered while using my methods. I may well not be able to help with problems that result from a combination of my procedures and various deviations from my procedures.

I advise that you make drive images after each successful step so that if something should go awry as you proceed, you don't have to start from the beginning, just restore your last working modifications and try again.

There are a number of web sites that will describe how these customizations can be accomplished during installation, but the procedures I will outline here are for modifying an existing installation without having to start all over again, or modifying a new PC that didn't come with an installation DVD, just a factory recovery partition.