1. First of all...I strongly suggest that if possible, BEFORE you undertake any significant manipulation of data involving your CURRENT HDD (1 TB?), you clone the contents of that secondary drive to another drive sufficient in disk-space capacity to contain the entire contents of your current HDD. The reason I state "if possible" is because you will obviously need another HDD of sufficient size to contain those cloned contents. It is ALWAYS wise that before making
*any* major drive configuration changes affecting the data on that drive a PC user should clone the drive's existing contents so that he/she will have an effective fallback position in the event things go awry. We perform this "ritual" as a matter of course, knowing from bitter experience that when one undertakes major PC configuration changes of one type or another the process can go awry. And when it does...it's comforting to know you have the means at hand to easily and relatively quickly return the system to its functional pre-problem-state configuration.
Obviously if you already have purchased the replacement 1 TB HDD (or will have momentarily) you can use that drive as the destination drive for the cloned contents of the "old" 1 TB HDD. I'll proceed on that basis.
2. When you fresh-install the Win 10 OS onto your new SSD it's best to do so with the SSD as the only installed drive. It's best to disconnect the current HDD from the system during the OS install to the SSD. If practicable, connect the SSD to the motherboard's first SATA port (connector) - probably the one the current HDD boot drive is connected to.
3. Naturally after the fresh-install of the OS you'll ensure that the SSD boots without incident and functions without any problems. YOU GO NO FURTHER UNTIL YOU CONFIRM THIS.
Check your BIOS upon bootup to confirm the SSD is first in boot priority order.
4. Working with the "original" 1 TB secondary HDD, (as you understand), you will need to fresh install whatever programs/applications are currently on the drive that you desire to be installed on the SSD (excepting the Steam games which I understand can be transferred. I'm not a gamer so I'm not familiar with the precise process. I assume you are).
5. I expect you will want to delete the files/folders related to the Windows OS on that HDD since they take up a large volume of disk-space on the drive and of course are unneeded in view of the OS being installed on the SSD boot drive. Unfortunately the Windows OS balks at a user's attempt to delete the Windows folder, even when that folder resides on a secondary drive in the system and not on the boot drive. When the user invokes the Delete command Windows will generate a "Folder Access Denied" message stating that "You need permission to perform this action". While there are ways for the user to manually take control of that Windows folder and delete its enormous number of files, the process of gaining such access is quite complicated and generally
very frustrating for most users.
6. As a general proposition I'm loathe to use or recommend a third-party (non-Windows-integrated) program to achieve an objective involving the Windows OS. I do so only when I've concluded that utilizing such a program is the only reasonable alternative in achieving the objective and that I'm reasonably assured whatever registry changes are made by the program won't play havoc with a user's current system and (hopefully) won't adversely affect the system in the future.
I mention this because I'm recommending a third-party program for your consideration in order to delete the Windows 10 OS from your secondary HDD (assuming you would want to).
7. A number of years ago someone came up with a simple-to-use program that achieves the objective of deleting the Windows folder (again, we're referring to the Windows OS residing on a SECONDARY drive in the system, NOT the boot drive). It's the "Take Ownership" program - see
https://www.tenforums.com/tutorials/3841-take-ownership-add-context-menu-windows-10-a.html
for a copy of the program. The website's description of the program and its use is quite clear.
8. Basically the registry change made in the OS will add a "Take Ownership" menu item that appears when you right-click on the Windows folder. After clicking that menu item the program will "mark" the nearly 200,000 files in the Windows folders in a manner that allows the user to "take ownership" of them. In so doing the user can delete all the files/folders in the Windows folder. It's pretty much an automatic process although during this process the user will be asked to confirm the deleting of this or that file. The process is not terribly quick but it's not unduly lengthy and there's very little intervention needed from the user during the process.
9. Obviously you can use the "Take Ownership" command for virtually any file/folder residing on the secondary drive in case of the "Folder Access Denied" message.
10. In any event, following whatever "cleanup" you've make to the "old" secondary HDD and you've determined that the SSD is functioning fine with the newly-installed programs, etc., you can then clone the contents of the HDD to your new 1 TB HDD and install the latter drive to replace the "old" HDD. Of course, again after determining all is well with the system and assuming you plan to utilize that drive for some purpose or other, you can delete all the volumes and/or format the drive.