yes this is totally normal.
That is the way it is meant to boot.
what happened the second time is that mint could see the primary drive with windows on it so it added it to the grub boot menu.
what also happened is that mint wrote grub to the MBR (master boot record) of the primary disk (windows).
so now if you use F12 and choose the second disk (mint) it may not boot.
but since you installed to it the first time grub is also in its MBR and so may boot it anyway.
well if you mix up the hard drives now you may have booting problems so please ask questions if this happens.