Ok, since you might get confused from the comments, I've decided to write an answer. Though what I'm suggesting is not a simple procedure and if you're not experienced enough, you will end up with unbootable linux, or even worse - broken partitions and lost data.
I would not suggest you to follow it unless you're experienced enough and know how to recover from boot failures later. I.e. booting from USB, mounting, chrooting, etc...
These steps are not a copy/paste howto, so if you have doubts or question on any of these steps, do not start with this.
- You can create one new partition (5GB for example) on your SSD and move some parts of your linux there.
- Then format it with Ext4 or whatever FS you prefer.
- Copy all folders except "/home", "/var", "/media", "/run", "/opt", "/boot", "/mnt", "/proc", "/dev", "/sys".
- Actually you should be copying "/lib*" folders, "/bin", "/sbin" folders, "/usr", "/etc" folders and some more probably.
- Then create "/sys", "/dev", "/proc" empty folders on the SSD.
- You should update the ROOT in your bootloader config and fstab.
- Here you should find a way to get the rest of the folders mounted, but since they are on a single partition on HDD, it's not that easy.
- you can mount them in /storage folder for example and make symlinks to the root fs.
- or mount them in /storage folder and then bind mount them to their root fs folders (mount -o bind)
- in both cases you should later update fstab to do the mounting.
Note: there are probably many other ways achieve what you want.
On my linux, I have everything on the SSD and a (HDD mounted partition) /storage folder to hold my /home/user/[some sub folders] /var/cache and some other data-huge folders with symlinks to the root fs.