The POSIFLEX issue might have to do with MBR. On your final linux installation, your partition table should NOT be using gpt but mbr and that might solve the issues.
It has to do with older BIOS not recognising gpt and henceforth being unable to boot from the disk.
N.B. you might have to configure your GRUB/systemd loader accordingly.
King Gizzard and the Jizzard Wizard