[RndTbl] can't stop md yet lsof is empty?

Very weird problem here: I umounted the filesystem on a md (RAID1) device. Then: mdadm --stop /dev/md1 mdadm: Cannot get exclusive access to /dev/md1:Perhaps a running process, mounted filesystem or active volume group? Huh? lsof /dev/md1 <nothing!> I remounted the fs and tried lsof <fs> and it was empty too. I can't seem to find any reason why mdadm --stop should not allow stopping this array. Any ideas? Anything other than lsof that can help find out why?