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Definitely more than a year ago I replaced the internal 1TB Apple SSD in my MacBook Pro (Retina, Mid 2015) with a Samsung SSD 970 EVO 2TB unit (and an adapter). Quick story, no problem so far - SMART stats say 47TB data unit reads and 27TB data unit writes.

Since a couple of days I'm seeing some sporadic failure in reading a few files. SMART "Media and Data Integrity Errors" count has increased. The problem is sporadic because if I test the file that triggered the error (e.g. by computing a MD5 from command line) I see "Device error" at first, but it then becomes ok after retrying.

Is it possible to understand how serious is it?

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    Yes there is: it is serious as any storage related errors are serious. Do you have a backup? If not make one and verify it is working, you can restore from it and it is kept up to date. the next thing is to run disk utility in recovery mode, and you may need a 3rd party disk repair tool, depending on the format of your SSD.
    – Steve Chambers
    Sep 22, 2020 at 13:16
  • Data loss is not a problem: I have 3 backups and a clone (verified). What I need to do is to decide whether to immediately buy a new SSD (I'm still in the warranty, but I suppose replacement times are long and I can't stay without for too long) or to wait a bit an see whether the problem grows or not. What I'm puzzled about is that macOS is not warning about anything, neither in log files.
    – Fabrizio Giudici
    Sep 23, 2020 at 14:52
  • I believe that reformatting a drive still maps out bad blocks (correct me if I am wrong) so reformatting the whole drive and then reinstalling might be a worthwhile next step, and as long as you keep up with your excellent backup hygiene you should be safe. Unless buying a new SSD is easily affordable and you'd rather be safe than take a chance
    – Steve Chambers
    Sep 23, 2020 at 18:16
  • Follow up, in case the information might be useful to others. First, I've filed an assistance request with Samsung and let's see what happens. Second, just to have an idea of what is happening, the "Media and Data Integrity Errors" SMART measurement rose from 772 at the end of September to 1157. That's about 6/7 per day. Most of times I detected the problem by means of a tool of mine that scans the files for computing a fingerprint; in half dozen cases, all in the past three weeks, I've experienced a system crash. It's clear that the unit needs replacement (with some doubts, see next comment).
    – Fabrizio Giudici
    Nov 24, 2020 at 8:37
  • I have five backups, two are clone of the system to an external SSD that can be mounted in my MacBook, so I'm ready to switch device any time. The only doubt that I have is related to a fact that I discovered this morning. I created a 'BROKEN' folder where I move files that have been impacted by the problem: in this way I make sure that the related SSD sectors are not reused. All these files have experienced the problem in a consistent way, I mean I could see a 'Device Error' retrying a read operation multiple times. With my great surprise, this morning I found they are now (follows...)
    – Fabrizio Giudici
    Nov 24, 2020 at 8:43

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End of the story: Samsung acknowledged that the unit was defective and replaced it under warranty.

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