Thank you for your input, but it did not help.
I could use the two older drives on either of the four internal sata ports and the array was working, so I think it's not a broken sata connector on the card. I made sure it's not the power supply or a bad sata cable either.
I still have the same problems when switching the jumpers to external and attaching the new drive there.
The pre-read of the new drive (attached to the motherboards esata) went to 70% without any problems, with the two older drives attached to the digitus card. Then I interrupted it to investigate the problem further.
While trying to locate the problem, one of my older drives got disabled (red ball). SMART has nothing unusual to report, so I think the best is to reconstruct the drive following this procedure, right?:
http://lime-technology.com/wiki/index.php/Troubleshooting#Re-enable_the_drive
After that, I will do a preclear on the new drive via the motherboard's esata port, and then replace the digitus.
I'm still not sure, whether this is a compatibillity issue, or a broken card...
The motherboards manual says, it has a "PCIe 4x slot @ x1 speed, with latch", and the controllers manual says PCIe x2 interface compatible with x4, x8 and x16. Could this introduce the problem? I always thought PCIe could handle these things, but right now I am not sure anymore...