Partitioning Scheme for SSD

Better leave all read-only files on SSD and use hard disk for everything else

https://wiki.debian.org/ReadonlyRoot should clarify which parts of the filesystem tree can be read-only: moving /usr to read-only would be the most significant thing (greatest space required) I guess but you have to ensure it is remounted as required when doing apt-get install or remove. See that page for more info.