Originally Posted by
Septentrionalis
I regret not being able to help you sooner; I hope this is still relevant to you and that you have not partitioned your drive.
The Steam client UI does not allow us to create parallel install folders for some reason, but you can create one manually and add it to the libraryfolders.vdf, which should be located in your "steamapps" folder. It should look something like this once you are done:
"LibraryFolders"
{
"TimeNextStatsReport" "bunch of numbers here"
"ContentStatsID" "another bunch of numbers"
"1" "C:\\Program Files (x86)"
"2" "C:\\SteamLibrary"
}
So the first two rows inside the curly brackets are specific to your installation, and you should not touch them. The number two is the new folder that you just created (here: C:\\SteamLibrary). If that does not work for you, we'll help you further.
EDIT: You will most likely have to restart the Steam desktop client to make it recognize the folder.