fsck.ext2: "dscheck(da8s2): b_bcount 6 is not on a sector boundary (ssize 512)"

[James Cook]

I just tried running fsck.ext2 from e2fsprogs on an ext2 filesystem of
mine.

All seems to go well until after "Pass 5: Checking group summary
information". Then on the console (in a brighter font) I see
    dscheck(da8s2): b_bcount 6 is not on a sector boundary (ssize 512)
and (dimmer again, so I guess this is fsck.ext2 speaking)
    Error writing file system info: Invalid argument

And the filesystem is not marked clean like fsck usually does: if I run
fsck.ext2 /dev/da8s2 again, it says "/dev/da8s2 was not cleanly
unmounted, check forced."

Any ideas what might be going wrong? I'm guessing from the error
message that /dev/da8s2 only allows sector-aligned writes, or something
like that... is that true, and it different in FreeBSD?

-- 
James