What approach do you want to choose ?
1)
To enable pruning simply navigate into the bitcoin data directory (default path:
~/Library/Application Support/Bitcoin/) and edit the bitcoin.conf file.
If it doesn't exist, simply create one with this content:
Replace
N with the size (in MB) you want to have stored on your hard drive. 550 MB is the minimum possible.
So, for example, if you want to have 5 GB stored, choose 5120 MB.
2)
If you want to keep the whole blockchain, move the
bitcoin folder to an external drive (again, standard path:
~/Library/Application Support/ to an external drive).
Only do this while bitcoin core is NOT running. Then start up core and it will ask for a data directory path. Point it to your new path.