Although it's possible to move images using Lightroom, it is generally more reliable to use your operating system to do so. The best thing to do would be to move entire folders of images from one EHD to another. When you do that, Lightroom will tell you that the images are missing because they have been moved from where Lightroom expects to find them. It's an easy fix. Here are the instructions:
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Jim has you pointed in the right direction.
Lightroom is not a file browser, and it tends to fall down when moving large numbers of files.
Do the file moving by moving the folders with your OS tools (Finder, Explorer, etc) then reconnect the missing folders in LR's Folders panel.
Thank you Jim.