Try downloading and installing it outside of Galaxy and see if that works (run the install as administrator).

I read that under Windows 8, the game had to be installed to a disk not labeled as storage? I don't have Windows 8 but.......

I quote;
"The game will refuse to run when installed on a drive which is managed by a Storage Space on Windows 8 and later. "
and;
"Installing the Game on a drive that is not a Storage Space will fix this issue."

This comes from the pcgamingwiki(DOT)com for the Jade Empire

Try running GOG and the installer as an administrator.

I know this is an old thread, but I have had this "Disk Access Problem" for hours, trying to download Jade Empire via Galaxy.
I tried restarting Galaxy, restarting in Administrator mode and Verify/Repair countless times with the same error.
I also completely uninstalled and tried to reinstall numerous times.

Eventually, I found this thread and tried the above advice to download and install outside of Galaxy, then run install as Administrator and it worked first time (though the configuration screen crashed on startup, but at least the game seems to run).

So, to anyone reading this who has this problem, try the above advice, it worked for me.

Then simply use the "More..Manage...Import Folder" under game options to add it to Galaxy