Just curious. - Internal HDD/SSD. By overwriting my previous OS (can be clean install or upgrade). (e.g. Snow Leopard) - Internal HDD/SSD. By making a new partition and installing it there. - External HDD/SSD. Erased HDD and just installed it. - External HDD/SSD. Repartitioned to not lose any data and installed on a new partition. - USB-stick/SD-card.