If you have the same hierarchy of folders, you may go ahead and update the parent folder location from Mac hard drive to Lacie hard drive.
To do so, please follow the below steps:
> Launch Lightroom
> Navigate to Folder panel in the Library module
> Make a right click (CTRL + Click) on the folder and choose update folder location.
> Navigate to the folder in Lacie hard drive
> Click on Select folder. Lightroom will update the new location.
Let me know if this helps.