I had the same problem and looked it up, finding this thread which was useful, including in knowing I wasn't alone! In my case I had imported nef files from SD to a PC laptop. I had then exported the files as a semi-edited Lightroom Classic CC catalog to a desktop PC. It was here that I noticed the problem on half a dozen files, one of which was important. After a minor panic, I went to the laptop, found them uncorrupted there and exported them as original nefs and reimported them to the desktop. Problem solved. Luckily I hadn't deleted the files from the laptop, however if it had been the transfer from the original SD cards and I hadn't noticed then not so good as they were formatted as soon as a second backup had been made. My conclusion is that somewhere between the laptop and the PC transfer of the catalog (via a hard drive), something has occurred in the actual transfer. I can't see how it can be anything other than a hardware problem (cable may have been poorly connected/damaged etc.). I've started running chk dsk - 14 hours to go 😞 - on the intermediary drive as it may well be a corruption on that, but for what it's worth - though I'm no expert - I'm fairly sure it's a hardware issue.
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