Hi chewitt,
First thanks for your support and your time. I have done this update of firmware like described but the situation was more worse afterwards, I could only see the first drive under every OS (Windows 10, Ubuntu 17.04, Pi) and with lots of errors in the logs. Fortunately I made a backup of the old firmware of Icy Box and restored it, now I have both drives again.
After I checked both drives with Ubuntu 17.04 (amd64 Virtual Machine) I remembered that the situation the case was running fine with my Pi months ago was with Ubuntu and not Raspbian. Sorry that was my mistake. Therefore I took a second SD card and installed the current version of Ubuntu Mate for Pi and it worked again, but as you can see on the first attached picture. /dev/sda and /dev/sdb will be mounted with the same device names but under different mount points. Afterwards I tried LibreELEC again and found also both devices under Filesystem and two times the mount 1.42.6-5644 like in Ubuntu. But when I would add this directory to the bibliothek I always can go in the first drive with "filme" and never in the second drive.
It seems that only this last differentation between the drives will be missed. Could I mount sda and sdb manually in LibreELEC or do you have a better idea for me?
Best regards
Steffen