Originally, I had Windows 8.1 pro installed in my PC having all the system/program files in my 'C' (SSD) drive and the 'Users' folder in 'D' (HDD) drive (as I had voluntarily moved it). As I had registered with Microsoft for Windows 10 upgrade, it was prompting me to upgrade many times. I had read that Windows 10 will not upgrade (fully and automatically) if the 'Users' folder is located in a different hard drive (other than the system/boot drive which is normally 'C'). I was hesitant for the last few days as my big 'Users' folder (>300 GB, bigger than my 256 GB SSD boot drive) was in 'D' drive. Then I read several posts in many websites to find a solution, and did get some ideas to complete the job in a few easy steps. In the following steps, I am explaining how I successfully installed or upgraded to Windows 10 (from 8.1) without doing any "relocate" option. This "relocate" option is very cumbersome (which involves copying the entire content of the USER drive back and forth) if your 'User' folder is too large (as in my case). So, I found a simple method to upgrade to Windows 10, keeping all the contents of my 'User' folder and drive intact (---this is the special thing about this method, which I could not find elsewhere in the related postings in the forums!!!). In the message below, I have used "BillGates" as my 'fake' account name, "C" as my 'system drive' and "D" as my 'Users drive'. Others who will be using this tutorial should use corresponding options as exactly used in their PC. [If anyone cannot understand this, then even the real "Bill Gates" cannot help him/her!!!]
I did the following as explained in 4-steps (actually 3-steps) to successfully upgrade to Windows 10. The process is very easy, and anyone can do it. It would just take 10-15 minutes even for a novice to do all these steps, and additional ~45 minutes (up to 90 minutes in case of a slow computer) for the entire upgrade process. It does not need special expertise if a person follows these steps carefully. Generally, although I don't come to discussion forums to contribute due to my limited time I thought I must share this success story which might be useful for many people with a similar situation. [Warning: Those who have not yet registered with Microsoft for upgrade (and those who are still waiting to hear from Microsoft), please do not proceed. Do these steps, only when you have the upgrade option ready as prompted by the Windows icon on the right-side task bar (if you had registered with Microsoft for free upgrade to Windows 10). Additional condition: Do these steps only if you have your 'Users' folder in another hard drive (as in my case). For others with a regular/normal upgrade, it is an easy process and these steps are not necessary and it may rather be counter-productive]
Step-0 (Optional Backup): I copied or backed up all important folders and files (in the 'Users' folder, e.g., Documents, Pictures, Music, etc.) in to an external hard drive (others may use any other choice/media of their own depending on data size, preference, and availability). This step is optional and not a requirement in case of a successful upgrade (as in my case). However, this back up will at least help you to have all your important folders and files intact for future use, even if the upgrade process is not successful or ends up in an unexpected problem (due to an unforeseen cause). Luckily, I did not face any problem from start to end.
Step-1 (Making two edits in the Registry Editor): From the "Start" button, click "Run" and type "regedit". Registry Editor window will appear. Go to HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows NT > CurrentVersion > ProfileList. [First Edit] On the right side pane, you will see some items (such as Default, Public, ProfilesDirectory, etc.). Right-click on "ProfilesDirectory" and select "modify" from the options. A dialog box will open. In the second field (value data), replace the letter (and colon) representing the original User's drive "d:" with "%SystemDrive%" (e.g., "d:\Users" will now become "%SystemDrive%\Users") and click "OK" button. [Second Edit] Now, in the same "ProfileList" folder click the subfolder with a long name of numbers "S-1-5-############" (I had only one long subfolder as I had a single user account, and I guess this may account for the number of users in the PC. So, one has to select appropriate subfolder to match with the account by just exploring each of the subfolders in case of multiple accounts!!). On the right side pane, right-click on "ProfileImagePath" and select "modify" from the options. In the second field of the dialog box (value data), replace only the drive letter from "d" to "c" as an example in my case (e.g., from "d:\Users\BillGates" to "c:\Users\BillGates") and click "OK". Now, exit/close the Registry Editor window. These two edits, as I understand, are allowing the existing Windows (8.1) to create a new (temporary and empty) 'Users' folder and its subfolders (Documents, Pictures, Videos, etc.) in the system/boot drive 'C'. Don't be afraid. All these folders can be later deleted (or, by choice, can be left as it is for the future upgrades as you can save one step), and all your original folders in the other drive 'D' will be intact and later become part of newly installed Windows 10.
Step-2 (Restart and Install/Upgrade Windows 10): Now, without doing anything else, exit all the running programs and "Restart" the windows. Windows will take a few extra minutes to boot and start up as it will create new temporary "Users" folder and subfolders in the system drive 'C'. Don't bother if the Windows warns you that "this is only a temporary User folder" or "your account profile information is missing" (or similar messages as I don't recall exactly, but not many messages!!). When you log in to your account (--use the same primary account throughout this process), you will see that this Windows (still the 8.1) does not look/feel like your regular Windows with your previous options/features (as you are now 'delinked' from your original User folder and features). Don't worry. Now, click the windows icon on the right-side task-bar (near the time clock) which was prompting you to update/upgrade. The upgrade process will start. Follow the instructions and proceed. This (upgrade) process will go on for about ~45 minutes with 3-4 automatic restarting. During the process of account setting (after about ~30 minutes), you will be prompted to set up or login with your Microsoft or Local account. Finally, you will see that the Windows 10 is fully installed. However, the windows (now Windows 10) may not look/feel like 'your' windows, as it could not access all the options/features specified in your original 'User' folder which is in another drive. Don't worry, and don't do anything else to meddle with the process (--and don't restart now!!!). Just follow the next step (which will rectify this problem).
Step-3 (Reverting back to original options in the Registry Editor): Now, without doing anything else, you have to revert the changes made to the Registry Editor in Step-1. Go to HKEY_LOCAL_MACHINE > SOFTWARE > Microsoft > Windows NT > CurrentVersion > ProfileList. [First Edit] Right-click on the "ProfilesDirectory" and select "modify" from the options. In the second field (value data), replace "%SystemDrive%" with the original User's drive "d:" (e.g., "%SystemDrive%\Users" will now become "d:\Users") and click "OK" button. [Second Edit] Next, in the same "ProfileList" folder, click the subfolder with a long name of numbers "S-1-5-############". Then, right-click on "ProfileImagePath" and select "modify" from the options. In the second field of the dialog box (value data), replace only the drive letter (e.g., from "c:\Users\BillGates" to "d:\Users\BillGates") and click "OK". Now, exit/close the Registry Editor window. Now, without doing anything else, exit all the running programs and "Restart" the windows. Windows 10 will open with all your original options and it will look and feel like 'your' windows, and all your original 'Users' folder and subfolders (in 'D') will be linked as it was before upgrading to Windows 10. Now, check whether all the directories and files (in your original 'Users" folder and subfolders in the data drive 'D') are intact. After your verification and satisfaction, you can now delete the unnecessary 'Users' folder in the system drive 'C' (if only possible at this stage; if not, leave it for now!!) although I suppose that this will not disturb you in any way other than occupying some disk space. You can now do one more restart, and then later you can go for any further periodic 'Windows updates' which we normally do.
Hope this helps many people in this situation. I should also say that I may not be able to solve any other associated problems with this issue. Further, I may not be able to frequently visit this forum either. Those who have complicated issues and problems, please post your messages so that some experts and technocrats will help you.
PS: My humble request to the Microsoft company is that Windows should come up with a built-in option which would ask for the locations (drives) for the system folder as well as 'Users' folder (maybe prompted by a drop-down menu) during the process of installation so that new upgrades would be installed seamlessly based on these inputs/options by the users (irrespective of the locations of these folders). Then, the customer/consumer/user need not go through such an Odyssey of reading through forums for this common and genuine issue ("upgrade problems when the User drive is different"). So many people are nowadays opting to use a small SSD drive as the system/boot drive (for various advantages including fast/easy running of programs) while using big-spaced hard drives as the 'Users/Home/Data' drive (for saving BIG data in a separate/safe drive rather than in the unsafe boot/system drive which is susceptible for data loss or damage due to any unfortunate program crashes, etc.). Therefore, it is high-time that Microsoft pitches in to help its users by optimizing the products (Windows 10 and future versions) to keep pace with the growing need of the time and trend, especially when even the free software/OS (e.g., Ubuntu) has such options during installations.