I installed smartmontools, then tried running the test ...
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So, sudo apt-get install smartmontools ?
Yes. Then follow some guide on how to check hard disk's health with smartctl, for example this one: https://www.thomas-krenn.com/en/wiki/SMART_tests_with_smartctl
I don't open the computer anymore. I have a tech who's cheap and willing. I'll try software options, first, then take it in, if needs be.
I still suggest checking or replacing the drive's cable. Hard drive read errors, including the ones reported by smartctl, are sometimes caused by a faulty cable or a faulty connector.