Fusion Drive with internal SSD and external RAID

Hi,

I have a 2014 iMac with built-in 512GB SSD and store most of my data on a WD My Book Thunderbolt RAID. The RAID is configured as stripe-set.

As I don't like having to manage the two separate drives, I tried to create a fusion drive using the SSD and the RAID set.

All works fine, so I can create the CoreStorage (diskutil cs create ...) and create a logical volume (diskutil cs createVolume) - but the system is never able to mount the new volume.
It appears to be mounted and unmounted within a second after creation, but then there is no chance to mount and use it.

I have seen threads where others created FusionDrives using similar setup but never exactly the same.
Any thought?

Thanks, Christian

btw this is on 10.11