Ok, so I opened up the computer and checked on the hard disk. It's a 2012 WD Caviar Blue. Not from yesterday, but also not 10+ years. Plus these were good hard disks in my experience.

So I went ahead and jumpered the hard disk von cable select to master and rebooted. And voilá - suddenly MorphOS recognizes the drive. I formatted it and right now installation is running through.

In case somebody else runs into this kind of trouble - check your jumpers! (thanks for the help, Kronos!)

[ Edited by Chefsessel 06.01.2018 - 09:52 ]