Lightroom CC2019: Hard Drive is full . How do I move storage SAFELY to external drive?

I am working in Lightroom CC 20190502 Version 2.3.  I have the all-app subscription with 1TB of cloud space.  I started migrating files from my Apple Photos library, with local storage set to the Internal HD on my iMac.  With 14,000 images imported so far, the HD is now full.   The solution to this in the Lightroom Help is "free up space on your HD."  So I switched local storage settings to an external drive with 4TB of space,  but the migration will not work because I do not have enough space on my Internal drive to complete the process. 

Is this a Catch-22?

I notice that most of the HD space is being used up by assets imported before I switched the local storage settings to the external drive.  Obviously, it would help if I could just move these earlier files to the external drive, but I am afraid of doing this using the OS system because it will probably break the links between Lightroom and the image files.  

How can I SAFELY move my files to this external drive, without losing the connection to the catalog? 

Thanks!