If you are encountering disk access error, when installing a game via GOG Galaxy, you should make sure that your security software (anti-virus, anti-spyware, firewall etc.) isn't blocking GOG Galaxy from openning ".exe" files. To do this, make sure that GOG Galaxy is added to the software's trusted applications list (whitelist). How to do this varies, depending on the security software.
Moreover, please make sure that you are using account with administrator privileges, and that you have unrestricted access to the folder you want to install the game to.