Moved Sessions now have images off line ... how do I put them back together

My had disk on my iMac was almost full, so I thought I made copies of each session and made a copy on 3 different external drives to free up space.

Now many of my images are "off line" when I go to my transferred Session. 
I purposely shot Sessions because I thought that I kept everything to do with that session in ONE folder. 

How do I rehabilitate my Sessions and off-line images?