Strange... the message itself comes from the underlying d-i component partman-efi. The logic it uses to present the message is "are there no EFI system partitions, but there are any other existing partitions". I wonder... there was some other flawed partition logic like this that was triggering on the installation media itself, so I wonder if that is what is going on here. In other words, the blank disk isn't what is triggering the message at all, but rather the fact that it is seeing a partition on the usb flash stick you are installing from.
Strange... the message itself comes from the underlying d-i component partman-efi. The logic it uses to present the message is "are there no EFI system partitions, but there are any other existing partitions". I wonder... there was some other flawed partition logic like this that was triggering on the installation media itself, so I wonder if that is what is going on here. In other words, the blank disk isn't what is triggering the message at all, but rather the fact that it is seeing a partition on the usb flash stick you are installing from.