Unmounting usb disk impossible

[Morten Bo Johansen]

Hi

I simply cannot unmount my usb disk. It is mounted by automount/autofs
on /media/da0s1. I try to unmount it like this

  sudo umount -At autofs

but it replies with the error message

  umount: unmount of /media failed: Device busy
  
I run lsof grepping for "/media" or "da0s1" both with and
without sudo. Nothing shows.

I got no terminals accessing the mount point, no zombie
processes or anything.

If the error message is really correct and some process is
actually accessing /media, how do I find it if lsof shows nothing?

Any other advice?

Thanks,
Morten