I experienced the same thing as well; that is, the built-in defrag sometimes doesn't do a good job, even for data drives. For example, I have one HD used to copy large video files, burn them to DVDs, then delete them. At some point, verification errors show up in the DVDs after burning, and they still show up even after I use another set of blank disks or optimize the HD and boot the system several times. It's only when I use a third-party defrag program on the HD does the problem go away.