Press Ctrl+Alt+F2 on Qubes installation screen to show console and type fdisk -l to list disk drives. Then you can fdisk /dev/sdX to select disk. Now you can type fdisk commands. Pressing Ctrl+Alt+F6 returns to install screen, or you can just reboot to see if your disk shows up. This should be the answer. This solves Qubes-OS being allergic to ... More @Wikipedia
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