Solved: To test if a computer's USB port is defective because an inserted item might fail to connect consistently, try using a USB-powered lamp (I use a gooseneck one to light the keyboard in dark surroundings). (Anything USB-dependent, not needing additional software support, giving an immediately-obvious result, moveable by hand up-down-left-right, and harmless if disconnecting will suffice for a diagnosis of this problem.) I often put a flash stick into the left port and a lamp into a right-side port. Everything was fine. But plugging the lamp into the left revealed intermittent operation unless I lifted the device by hand. Probably the port defect didn't much affect the flash because the flash weighs much less, but it explains why my software sometimes implied the thumb drive had loosened although I didn't sense looseness with my hand. My cheap and already-useful USB lamp has become more useful. I think it cost me $5-10, new; online it's likely less.
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