Relocate music from Macbook to external Hard drive

Hi guys! 

I've found some topics with quite the same problem, but I've never seen a 100% correct answer for my problem. 

So, here it is: 
I buy all my music on iTunes and I place all my free downloads to the same folder, called 'Purchased'. 
Afterwards, I move all my music to Rekordbox where i've made different folders for all kinds of genres. 

Problem: my Macbook's memory is full, so I'd like to move all my music to an external Hard Disk. How do I do this, without changing (and replacing all music) all my folders in Rekordbox all over again? Just looking for an easy way for this! 

Please tell me the solution for this! :-)