Yesterday my HDD finnally gave up. Lately the notebook became slower and slower and the HDD louder and louder. After opening the small lid on the right side, I realized that there is no 1,8″ HDD built in. In fact its a normal 2.5″ but for replacing the HDD the whole notebook had to be disassembled.
The second difficulty was that the HDD is not a normal 5V no it is a 3.3V which you can only get through OEM support. I have no guarantee and anyway it will take too long so after a while I found a guide how to tweak a 5V HDD to become a 3.3V. It was quite easy I replaced the 40GB HDD with a new Hitachi 160GB, broke the 2 pins as described in the guide and the Computer recognized the new drive 
The only bad thing which I have to figure out now is how to clone the old HDD so that it also takes the hidden partition to the new disc. I tried with Acronis but the software could only see my normal partition. Anyway at the moment it is only important that my computer runs again and for cloning etc I got an external CD drive. So it is not that difficult to install an OS without that hidden partition.
Another good source for the R3-5 I also found on my quest.
