On Sun, 3 Jun 2018 at 14:22, Peter Lesterhuis <peterlesterhuis@telfort.nl> wrote:
> And you won't mount at /mnt/sda2, you'll mount at /home, /usr /tmp in
> order to do what you want. You'll have to create those directories
> under your new partition, and make sure they have the correct
> parameters (the same as /usr and /home have under /).
Thank you, and others, for replying.

I learned from you that /usr and /tmp should be on the SSD-drive in
order to have maximal profit from this fast drive.

They are needed to boot.  If there is an issue with your rotating drive, it
is very helpful to have a system that can still boot.  
 
Now I would like to have only "/home" on the HDD. This is the folder
which is going to contain large files (video, musix, etc).

I created a new partition on the HDD, mounted it (there is a line now in
fstab).

During the installation /home is now on the SSD. Can I make a symlink to
an new /home at the HDD-drive?

What would be the right command, if the mountpoint of de HDD-partion is
/mnt/linux-data

Would that be: "ln -s /home /mnt/linux-data/home"?

Not quite.  Rather than asking for recipes to follow blindly, you should
teach yourself by doing some simple examples.  Start with 
https://en.wikipedia.org/wiki/Ln_(Unix) for background, then make
up some exercises to get some practical experience before making
changes to critical directories or files.

(Seems far to simple).

Or is the idea of symlinking not right.

Symbolic links are very easy to use and can be very helpful, but 
do sometimes cause confusion when things like "cd ../../" put you 
in an unexpected place.   Symbolic links have been around since 
before linux and should work the same way on all linux and unix systems.

Some years ago, unix workstations from companies like Sun and SGI
came with relatively small system disks, and were often used with much 
larger external disk arrays to hold data.   Typically, users would have a home
directory that contained startup scripts and application configuration
data.  Thery would also have symbolic links to directories on the 
disk arrays.   The same sort of approach is useful today with small
SSD system disks and arrays of rotating storage.

There are tools that will show the total space used by directories.
You may be able to move a few big directories to rotating storage 
while preserving most of the current home directory on the SSD.

--
George N. White III