>There's a bigger problem: it is possible to tell a disk to write something somewhere, and it reports success, but has actually either written something different, lost it entirely, or overwritten something different
My ZFS experience on a ~5TB pool consisting of six commodity HDs under fairly light load (ie. it's a home file server) is that every couple of months scrub detects checksum errors in a block or a small handful of blocks, without any corresponding read/write errors being given by the device.
Not sure if that's the situation you're talking about.
(Also, the same experience has taught me that Western Digital should be avoided like ebola. I actually wonder if their green series might be drives that have failed QC and been re-badged for the unsuspecting consumer.)