Question Problem with windows boot drives

May 12, 2019
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My brother's computer for some reason now just tells him to insert proper boot device upon turning it on. I haven't been able to fix it so I gave him my boot SSD to use on his computer. This worked. So his SSD is the problem! Right? Well then I tried his SSD on my computer and it worked. So for whatever reason his computer just won't load with his from his SSD. It also won't boot from a USB drive despite being a new pc.
Any help would be very appreciated thanks.