If you're not planning on disconnecting the old drive, you can simply do what I do to relocate all my Steam games to a non-Steam folder.

-Move the game to your desired installation folder with good old-fashioned copy and paste.

-Open a command prompt as Administrator. Google will tell you how to do this if you don't know.

-Enter this command, without quotes and consulting the following notes before entering:
"mklink /D <x> <y>"

Replace <x> with the old/current FFXIV installation folder, enclosed in quotes.
Replace <y> with the new/desired FFXIV installation folder, enclosed in quotes.

For example:

mklink /D "C:\Program Files (x86)\Steam\steamapps\common\FINAL FANTASY XIV Online" "D:\Games\FINAL FANTASY XIV Online"

It creates a symbolic link, like those found on Linux operating systems. I use this to shove all my Steam games into a very specific folder without triggering the Steam DRM.