There's another way: Directory Junctions
You can have a folder on one drive appear & act, for all intents & puposes, as though it actually exists in another location/drive.
Lets say your DCS is installed in D:\DCS World OpenBeta\
So Syria would be normally located at D:\DCS World OpenBeta\Mods\terrains\Syria
If you want to move it to another drive, due to lack of space, let's say E:\DCS\Syria
run these command:
mkdir E:\DCS\Syria
mklink /j D:\DCS World OpenBeta\Mods\terrains\Syria E:\DCS\Syria
When you get a success message, just download/install Syria map, and it will actually be written to E:\, but will appear in D:\
If you're not comfy with cmd commands, get this: https://m.majorgeeks.com/files/details/hardlink_shell_extension.html