When I install to a Hard Drive, it says it is out of disk space even though I have enough space on my Drive.
Are you using Galaxy?
I have the same problem from time to time. Seems as Galaxy becomes bitchy when the disk space is "low" (although there is still enough space). I freed up some space to make it work in the past. If this is not possible for you, maybe you just try downloading the game without galaxy, install it and import it to galaxy. Let me know if that worked, could be helpful in the future!
Or much easier solution: GOG OPEN SOURCE GALAXY!
In this case people like us could fix this on our own...