you need 70G of space to unpack the patch, no more, no less.

it doesn't matter if you got the game on Steam or Gog because its the same patch and running software that blocks access to the C drive on your system = what a shock, no space error

I get the E8 error when I try to install to my 1TB HD. But I don't have any problems installing to my smaller 500GB drive.
I have do enough free space on both drives.
I tried this with a number of games that I own on GOG. I get the same result.
Perhaps, the size of the target disk is a coincidence, but that is my experience.