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Vvei_61's post in cache pool errors - failing drive or ?? was marked as the answerYour drive is failing. The self-test passing doesn't mean it's healthy – it only checks if the drive responds, not whether all sectors are readable.
Look at the SMART error log: 399+ Unrecovered Read Errors, all pointing to the same LBA range. That's permanent bad sectors. Back up your data now, this can fail completely without any further warning.
Frank