Imported Images into Existing Session and Now They are GONE

I created a new Session and tethered the session. A week later I did another shoot and tethered the new Session to the previous session since it was for the same client, so everything was in the same Session. I color labeled my favorites between the two shoots and everything was working great. Soon, the folder was taking up too much space on my Desktop, so I transferred everything over to my External drive. I try to go back to the Session, which I had to find the cosess file for, only to find only the images from the first shoot are within the session! I'm missing the entire raw files from the second tethered session! D: What did I do wrong and how can I fix this?