This question already has an answer here:
The issue is basically this - where should I mount SSD and where should I mount the HDD if what I want is the system to be installed on the SSD, but new software installed through apt-get and such will be installed on the HDD.
Assuming this setup is possible of course.
This answer suggests moving parts to the HDD is possible but does not specify how.
My specific case is I've installed ubuntu on a multiboot machine, mounting a 10GB ssd partition on root(may be a bit too small in retrospect), and HDD on /home. But it seems I'm filling up the ssd partition quicker than I wanted as everything seems to be installed there.
This question is on the same topic but does not mention where applications are typically installed and does not suggest where to mount HDD for new applications to be installed there.