Remove the game installation completely, including the: This PC > Documents > My Games > Final Fantasy XIV folder. However first, you best copy and paste this folder some place else, as it has all of your settings such as macros, setup settings, character related stuff.

Once its uninstalled, install it to the C drive if it wasn't (OS location), if it already was, then you need to try a different drive on the computer, or even better, attach a removable USB storage device with over 80GB of space free and then test it.

Then you can rule out the default drive being the issue.