Thank you for all your advice :-)
Hereby my update:
I received help from Synology and this was there reply:
Thank you for contacting Synology Support.
If you were using volumes with data protection (i.e. RAID1/5/6, which allows 1 or 2 failing disks), you can install DSM on a spare empty disk and then insert the remaining hard disks to read the data. (Please be advised that this is still risky. If one more hard disk fails during this procedure, then your data may be lost). Here are the steps:
1) Power off and remove all hard disks
2) Insert a spare empty hard disk in Slot 1
3) Power on and install DSM normally
4) Do not create any volumes. Power off DiskStation once DSM is installed successfully.
5) Insert the remaining old hard disks while keeping new spare disk in slot 1. (1 of your old hard disks may be left out.)
6) Power on DiskStation
Once the data in the old disks can be read, you should back them up to another place asap.
Or, you can go to Storage Manager and repair the system partition on the old hard disks. When it's done, you can remove the spare disk, install the original disk, and repair the system partition on the original disk. By doing this, your previous system configurations (user account, user privilege, application settings, etc.) will be erased, but the data in the shared folders will still be there.
If you need further assistance on this issue, please feel free to contact us again.
I bought a spare 3TB hard drive and installed the DSM 4.3 (so the one I previously used).
After that I had put all hard drives back in, restarted the NAS and all my data was still there :-) I was happy as kid.