Hi guys, thanks for all your feedback. I now switched to Seagate ST3000VN000 because I wasn't to interested in getting another set of WD's after I read some fun reviews on Amazon of other people running into similar problems at around the same time after having bought them in the same place.
Instead of resetting the system again, I added one of the 3TB disks to the 100GB drive that the NAS was running on and it worked nicely. After around 40 minutes the NAS was 'redundant' again.
Currently the second drive is inserted as well and I am a couple of minutes away from being back at redundant again with a 2x3TB configuration in X-RAID Raid1 mode.
Not sure if I will keep the RAID1 setup, but still, the system is running and unless something goes wrong in the next couple of minutes or the system will not reboot, I am happy.
So thanks again