Whether he notices or not doesn't mean there isn't a degrade in performance.
Solid-state drives are different from the mechanical, magnetic hard drives in wide use.
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What i would have done with that external drive is open it and look at the usb to sata bridge. Most likely the ssd can be removed and used as a sata drive, much faster than a restricted usb ssd. If that was possible, i would install Windows on the ssd so games benefit faster pagefile, driver and directx initiations. Id wipe the hdd and use it for games and general storage. Ssd with Windows installed could have 1 or 2 games depending on size.
Point being, Windows being on the ssd would be so much better, especially since you only have 8gb ram. Windows consumes a bit of...