No idea about how to or why but tonight my gt 31 found the card and is running
the only new thing I did (compared to everything I've already written about) is that I closed the cover of the sd card...
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I unfortunately have the exact same problem. I have been using coros vertix for last 3 years but wanted to use my old gt-31 again after 3 years. I still have 2 gt-31... one finds the card . The other does not, when I change the card the problem stays with the same gps. So is definitely not the card but the gps. What is the best product for cleaning those contacts
