My C drive is not big enough to hold a DLC, but the main game is installed on this C drive. Is there a way I can force the DLC to be downloaded to a different drive?
No, you can't, because DLC is an add-on to the base game. In order for the game to know where the DLC is, it has to be installed to a predefined location, ie. the game's install folder. That's why Steam doesn't ask you where you want to install it.
If you want to install the DLC to a different drive, you can configure the drive as a game location, move the base game over, tell Steam where the new location is, and then download the DLC. That will solve the problem.
There might be a solution, but it ain't a pretty one and is not guaranteed to work. I also understand that it is not a direct solution, because it does require enough disk space, although temporarily.
You can try to make this work through symbolic links (I assume you use Windows for this). Here is how to do it:
- Make sure there is enough space on the drive where the game is installed on to install the DLC.
- Get a list of files of the game (potentially by copying everything temporarily, or use command line)
- Install the DLC
- Check which files were added
- Move the added files to your preferred location
- Create symbolic links for the added files
- Make sure the permissions are correct so steam can access the files
Be prepared to encounter errors, this does not always work correctly, especially when other files are changed instead of added. If this does not work properly, try to move the files back to the game directory and remove the symbolic links. After that verify the game files through steam.