The boot order setting of virtual box only allows to choose boot order between different classes of devices, like hard disk, optical, floppy, etc.
What I can do is to hit F12 during boot and choose one of the hard disks from the boot menu that will appear, yet I have to repeat that procedure at every boot. How can I permanently change the boot order among multiple hard disks until I want to change it again?
Changing the attachment order of hard disks is no usable solution as it will change the boot order but it will also change the hard disk naming in the booted system. On a real PC I can choose from which hard disk to boot in the BIOS and this setting sticks permanently.