From what I understand from recently looking into buying a new micro SD card for myself, that's pretty normal. I was searching through Amazon comparing the different 64GB micro SD cards and reading the reviews and came across a guy who wrote a review explaining why this is. It got way too technical for me so I can't recall exactly why. But he claimed that it's normal for any brand of micro SD to have read/write scores way below what the manufacturer prints on the package. Anyway, he seemed to really know what he was talking about and he recommended the 64GB Samsung Pro (I think that's the name of it, there's the Samsung Evo and then it's the one a step above that), hee claimed that was the best one to buy. From what I remember the SanDisk and the Samsung 64GB cards are pretty similarly priced as well. It makes sense, a Samsung Micro SD card working the best in a Samsung product I guess. Maybe try to return the SanDisk and give the Samsung a try?
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