LOW HD space - Can't install -- BUT, I use TWO HD's, and HAVE extra free space on C:

[76561198123043133]

I've got an Alienware setup, that came with:
Drive C: -- Solid State 100 GB HD for drive C, system drive
Drive D: -- 1,000 GB DATA DRIVE - Magnetic conventional spinning Drive D: 1,000 GB /1 TB, 500 GB is FREE.

I need to FORCE Steam to TRY to install WAR THUNDER, since it "only takes 50 GB". My Steam App FOLDER is VIRTUAL, and set up to automatically install on a folder set, on Drive D:, using MKLink Windows Command.

-- Another option would be to SPECIFY which DRIVE and FOLDER to use when installing, and then copy all these files to either D: or C:. (They would work AS Drive C: folders to steam).
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Obviously, my space on Drive C: System is very limited, and generally only has 5 GB free, if I'm lucky. In the past, when I've run out of space, I've moved some of the programs I don't use very often from Drive C: to Drive D:. However, unless I "fool to program", into thinking the files on D: are really on C:, they won;t run. So, that's why in Windows 10, I have used the MKLink command.

Specifically for STEAM, I first had Steam installed on Drive C:, and installed RaceTrack, Pinball, and a few other programs. Then, I COPIED over the largest complete folder SET of all the subdirectories and the files, FROM drive C: TO Drive D, using identical folder names. (I didn't have to use the same ROOT folder name I created on D:, but it is much less confusing if you do.)

Then, from a Command Line running in ADMIN account, I ran THIS command:
MKLink /J "C:\Program Files (x86)\Steam\SteamApps" "D:\Program Files (x86)\Steam\SteamApps"

Next, I Opened Steam, and tested running RaceTrack -- It ran, So then I shut down Steam,
Next, I RENAMED the Main CRITICAL RACETRACK folder to something else, ON DRIVE C:

Then, I Re-Opened Steam, and tried to run it. Steam AND Racetrack RAN FINE! I used the same method for a few other large folder sets, and saved about 200 GB that way.

--- PROBLEM --- I'd LIKE to install WAR Thunder...
But, I do NOT SHOW that I have 50 GB free on my REAL Drive C:, even though I could add 500 GB of files and folders to my VIRTUAL folders, on DRIVE D:. (Also, when I INSTALL these on Drive C:, they automatically are really installed on Drive D:. i.e., I haven't touched this setup in years. Recently, I installed BEJeweled 3 fine since it was small (<3 GB), and that required NO space on Drive C: after I installed.)

Let me know how I can overcome this limitation. An override, forcing it to seemingly run out of space is one way, and the other would be to SPECIFY which drive/folder to use for installation.

[Bad 💀 Motha]

What's the issue. Configure a Steam Library folder on D Drive via Steam Settings Downloads and select it during a game install. It's that simple, always has been.