Some software/game like Oculus or here Reforger like to force download file inside the HDD/SSD where Windows is installed, and don't let you choose a specific installation folder.
Yes it's pretty frustrating, but of course there's a simple fix to this.

What you want to do is a "symbolic link" from a folder in C:\ to a folder in D:\, what it does is simply make your pc think that it will install file in C:\ but in fact all the file are gonna be installed in D:\.

You can see sort of an analogy with game shortcut you have on your desktop, you click on an icon on your desktop that is located in C:\User\desktop but it will execute a .exe file in D:\Programm Files\Steamapp\YourGame.

!! Read fully each step before doing it !!

1- Locate where the mods are downloaded, i think the path is "C:\Users\User\Documents\My Games\ArmaReforger\addons".

2- Delete all the mods inside "addons" or move them in a temporary folder if you don't want to Re-DL every mods you have.

3- Delete the "addons" folder located in "C:\Users\User\Documents\My Games\ArmaReforger".

4- Create a new folder in your D:\ directory where you want the mods to be, here i've named it "FolderWhereYouWantTheMods".

5- Open windows CMD as admin and enter this line of command :
mklink /D "C:\Users\User\Documents\My Games\ArmaReforger\addons" "D:\FolderWhereYouWantTheMods"
there is a space in between like this : mklink /D "C:\..." "D:\..."
it will create a folder "addons" with a symbolic link to "FolderWhereYouWantTheMods"
You should have a message saying the symbolic link has been made.

6- If you didn't delete all of your mods it's time to put it back in "D:\FolderWhereYouWantTheMods"

7- Enjoy

"mk" stand for "make link"
"/D" means that you want to create a symbolic link, which is a "Directory" type and not just a file.

If for some reason you can't or have issues making a symbolic link, you can try a easier fix made by my boy Ben'wah.

https://gtm.steamproxy.vip/sharedfiles/filedetails/?id=3364871203