So let us summarise and wait for fvdw to help:
I have to discs setup:
One WDC20 with fvdw 15-1
One WDC25 with fvdw 15-3
when only one disc is plugged in slot 1 it is all OK
when two discs are plugged (slot1 and 2), the webinterface shows fvdw version installed in slot2 -> ergo it is booting from slot2
the file structure in folder /etc is from slot2
In the webinterface under disc setup, setup, manage disks and volumes: Disk-A is the size of the disk in slot 1, Disk-B is the size of the disk in slot 2
Can this be a cable issue?
Remark: If so, I can dismount the housing and change it, rather than having a patch that I have to trace on every update.