That's what I thought: your phones have a memory problem. I would suspect you swapped a memory card from the old Note 9, which was giving you trouble, to the N20U. But, since you say it happens with internal memory as well, there's obviously more to the story. One possibility is just plain bad luck, and you got two phones in a row, both of which did the exact same thing, whether through faulty software or hardware. That's a real long shot, but "never say never." Another possibility is that the phones have been unwittingly subjected to environmental conditions which damaged them. A third possibility is that the phones haven't actually been damaged, but environmental conditions are damaging the files stored in them. The fact that you're experiencing the problem on two different phones, on both internal and external memory, leads me to one of the latter two explanations. I would have to know more about software versions to even consider the first, and if it is a software problem, Android 11 should fix your issue soon (I recommend you factory reset the phone when you upgrade it). I seriously doubt it's a hardware problem, because it happens on both internal and external memory. In the meantime, the best way to avoid losing your files is to back them up manually as soon as you take them. I use One Drive, but Google Photos or any other will work as well. The other option is to try to send it back to Samsung Repair, but tell them to keep the phone as long as needed to replicate the problem. I would have returned the phone within 30 days, but you seem to be having the horribly bad luck that your phone worked fine for the first 30 days.
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