I found simple solution!
The external LG BE12 drive has USB2.0 and eSATA interface.
On the no more available Windows PC I always used the eSATA interface, which doesn't exist on the iMAC 24".
Accidentally I found an eSATA to USB-C cable offered by the large online store and it was quickly delivered just today.
Using this cable the DVD-RAM is recognized by the iMac 24" and the large *.VRO file can be downloaded from the DVD-RAM.
Easefab converts 4GB *.VRO into 2GB *.MOV within 12minutes and iMovie loads the file for further processing.
So finally, I have no idea why using the eSATA interface made the difference for the DVD-RAM, as DVD+R worked with USB2.0.
Nevertheless I don't need to know why, it just works now!
Thanks to everybody who might have thought about the problem and the solution might help others ...