For the surface area of a 2.5" HDD, a cosmic ray penetrates through that and can potentially "flip a bit" (change a one to a zero or vice versa) every 8 seconds. Its a minor change, but it is real and documented. It becomes a problem cumulatively, and after enough time passes (after enough cosmic rays penetrate) it can corrupt a file.
There is no real way to shield against that, because cosmic rays penetrate everything; its not that they are so powerful as to brute-force their way through (on the contrary) it is more that they are so tiny that they can avoid hitting most molecules or atoms in a solid object, sort of like the way objects in space are so far apart that they seldom collide.
In a PC it is more a factor of memory leak, which is an app design flaw, common to Windows PCs (but mostly cured in modern versions of the OS).
3 days now, and numbers agree; seems like I made the right move here.