Unless XLN Audio has changed this, you can’t move it’s content (sounds) after the install 
…you’re going to want to uninstall AD, making sure there’s no traces of it left-over, then when reinstalling, put the program itself onto your OS HDD in the proper VST folder, and put it’s sampled sounds onto your 2nd HDD. Be sure to pay attention when installing that you do this correctly.
I couldn’t know what else may be wrong on your system, but using a multi-hard drive set-up is a must-have imo. You do not want a single HDD trying to keep up with such demands. This is a proper start at least out the gate.