Disconnect all harddisks except the future system drive. This way you avoid an accidental install on a storage drive (which will be configured after installation anyway).
But I was not used to this unusual installation method different than usual Linux installations.
But yes, one should indeed read the installation instructions (most people read it as much as they read EULA: they do not. #TLDR)
Whatever, I will not touch OpenMediaVault for now (I burned it onto a rewriteable DVD. Let's overwrite it.) I will just use a SMB server.
I just needed it for one purpose anyway: Bypassing tedious MTP and USB OTG when transferring files from a mobile phone to free up space.