ZFS WARNING: Cannot open ad4s3d for writting; how do I clear it?

[Yoshihiro Ota]

Hi,

I have running FreeBSD 7.1-RELEASE-p14.

I had had a ZFS partition as one of data area on a system and
replicated a system using dump/restore.  On the replicated system,
I don't use ZFS and ad4s3d is mounted as /usr.

I get "ZFS WARNING: Cannot open ad4s3d for writting."
each time I run "zfs import" on relicated system.

How can I clear this meta-data information ZFS carries?

Thanks,
Hiro