I changed the settings in BIOS, no need to change anything on the installer (media). The install wizard detects the drives correctly. I still needed to set the mounting pount of sdc (where Ubuntu was to be installed) as well as (important!) the bootloader set to sda.
I can confirm, that it affected my system as well (see https:/ /bugs.launchpad .net/ubuntu/ +source/ grub-installer/ +bug/1769732) trying to install on a non-UEFI system.
BIOS settings for USB-Boot were set to UEFI-Boot, though.
I changed the settings in BIOS, no need to change anything on the installer (media). The install wizard detects the drives correctly. I still needed to set the mounting pount of sdc (where Ubuntu was to be installed) as well as (important!) the bootloader set to sda.