The Software Update service stores its package files in a static location at:
/usr/share/swupd/html
These files can take up gigabytes of hard disk space. If you want to save space on the system volume by storing the updates on a different volume, it's possible to use a symbolic link to point a location where packages reside on another volume.
Note:If you have previously enabled the software update service, you need to delete the existing /usr/share/swupd/html folder before creating the symbolic link. For more information on creating symbolic links, see the ln man page.
For example, if you had a volume named "ScratchDisk" where you want to store the updates, you could create a symbolic link to it with this Terminal command:
sudo ln -s /Volumes/ScratchDisk /usr/share/swupd/html