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Scrub of one external USB drive took 2days-3hours. Scrub of the second one is still going...
No, the purpose of scrub is explicitly to look at problems with the disk that happen "spontaneously". Two examples of such problems are mechanical damage to the platter and head (scratches, foreign objects, moguls in the lubricant), and off-track writes (where data should have been to one location, and ends up in a different location). These problems don't just happen to the data that is being written.Well I was doubting if it only checked the "meanwhile changed datablocks".