Three new USB stiks kept on de-registering

Hello. Back in the summer, I mail-ordered a respected-branded 16GB memory stick which from new had a fault that caused it to de-register & register at random, even if not mounted. So I sent it back for a replacement. The replacement did the the same but increasingly less frequently, and has not now done so for a long time. I didn't send it back because I didn't want to risk having the supplier think that I was being fraudulent (having read about a bloke who was arrested for sending back a pizza that was delivered cold). Then a few months later (recently) I bought a different well-respected brand 4GB memory stick in a local store and it too did the same thing - incessantly AND ALSO caused my machine to re-boot - twice. When I took that back, the manager tried it on his Windows machine for a while and it was fine; - - Very - - embarrassing. He would not replace it but refunded me, saying that it was my machine that was "out of date". Besides all of those I have for a long time had two other memory sticks which have worked faultlessly all along on my machine - so I know that the machine is OK! I run Knoppix mostly [or else Ubuntu] which is why it is easy for me to see the sticks register & de-register. Sorry if this is the wrong forum, but has anyone experienced this problem or heard about it?

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