I’m doing a rebuild on a backup drive, where a strange thing happened. After extensive testing, narrowing down where the problem was, an “fsck.ext4 -a” couldn’t detect a subtle, depeer issue. To detect the issue was brutally difficult, because the drive also got clean bill of health from smartctl as well. I reformatted it to BTRFS, to afford me scrubbing. It was only during a scrub that I could see the deeper issue - a massive slowdown in scrub speed partway through the scrub.