Actually, all (or almost all) spinning rust disks buffer writes, unless they're instructed not to, which is normally done by hardware RAID controllers. In that

Actually, all (or almost all) spinning rust disks buffer writes, unless they're instructed not to, which is normally done by hardware RAID controllers. In that case, the HBA takes over the job of buffering writes, and can safely acknowledge them to the OS as flushed before the write ever actually touches disk, because the writes are stored in battery-backed or otherwise non-volatile memory. It's well known in communities that care about storage reliability (such as the database world, which is where my experience comes from) that consumer-grade drives, in particular, are notorious for lying about writes having been successfully flushed to the underlying media.