Question that may affect what method you choose.
Do you intend to use this Library (ries) on different computers?
If Yes: Calibre Portable. That is self contained wit the Configuration stuff included. (desktop, keeps that per logon user)
1)
DO NOT USE ADD for either mode just to
Move to another (always an ATTACHED) drive.
2)With Calibre SHUT DOWN.
Simply COPY/Move the Calibre Library FOLDER (or what ever name) TO the drive (The folder name IS the Library Name). Start Calibre and use Create Switch: USE EXISTING to point to the new location.

Use Windows Disk Management (right click the Start Button)
to set it. Change the value to
L (for Library suggested) and set the range to the same. DO NOT use/change the letters of any system drive (C, D)

The next (and every) Time that drive is attached, to that system, it will be assigned L:
If you go 'The Portable' Route:
Simply Install Portable. Then locate
Calibre Library FOLDER on the Portable drive, then Replace that with the old one. Mostly done

You might want to copy the
entire Configuration folder CONTENTS to
Calibre Settings on the Portable installation folder .