Hi Folks - I got it! I found the SSD finally and how to replace it. It is incredibly hidden deep inside and it is not a 'normal' SSD as you buy it to replace a HDD but a special M.2 2260 type, hardly recognizable on the bottom side of the motherboard. It is replaceable and further down I will tell you where to get a 500 GB SSD for under 200 Euros to replace it.
Here is how you get deep down to the SSD - be very careful not to damage any cables, plug-pins, etc. - this stuff is very frigile and it is quite a challenge:
First disassemble your machine like shown in this video (you don't have to take out the HDD of course, just to show you how to get to the motherboard):
https://www.youtube.com/watch?v=MctmOvfm...
In the video a slightly different model is shown. The only difference to ET2702 is the different wireless antenna, which has to be detached here, best with a small screwdriver or so, be gentle!:
Indeed as risleh says the SSD is on the bottom side of the motherboard. To get the motherboard out (keeping left edge down, just tilting) take out these screws (circles in following image) and detach these cables (rectangles):
Then only tilt up the motherboard carefully since there are still cables attached on the left side. The SSD is at the bottom right:
and here a closeup:
As said, this an M.2 type SSD. M.2 SSDs come in different sizes. This one is 60mm long so it has the so called form factor 2260. There are also 2280 and 2245 SSDs on the market (80mm and 45mm). But in this case we need a 60mm size.
Here is where you can get one (500 GB):
https://www.ipc-computer.eu/notebook-spa...
By the way: this site was recommended me by ASUS support! There is a second ASUS parts site: http://www.asusparts.eu/en, but there I couldn't find the right SSD.
And when you got your new SSD, here comes the software part:
Clone your existing system on the new drive like shown in the following video. You do this by only using Windows tools. You do not have to buy or download any cloning software. The only thing you need is a USB stick.
The steps are:
1. Make a disk image on the internal 1GB HDD (if not enough space there use an external USB drive with enough space instead)
2. Prepare the USB Stick as a recovery disk (as shown in video below).
3. Replace the 128 GB SSD by the new 500 GB SSD physically
4. Boot from the USB stick
5. Copy the disk image onto the new 500 GB SSD
I haven't done all this myself since I'm still waiting for my new SSD to be delivered, but it looks easy in the video:
https://www.youtube.com/watch?v=Lh-Cd4vx... (sorry, video in german, but sure there are a load of similar videos in other languages).
Cheers and good luck from MarkU, Zurich, Switzerland