resize freebsd slice

[Giorgos Tsiapaliokas]

only /home is in a diferent partition,but i don't care because i don't have
any file in it (only .xinitrc,but this is not a prob)

i give as a root the command "dump -0Lauf /mnt/hd/FBSD/ /dev/ad0s3a"

but i receive the following error:

  DUMP: Date of this level 0 dump: Sat Jun 12 01:39:31 2010
  DUMP: Date of last level 0 dump: the epoch
  DUMP: Dumping snapshot of /dev/ad0s3a (/) to /mnt/hd/FBSD/
  DUMP: mapping (Pass I) [regular files]
  DUMP: mapping (Pass II) [directories]
  DUMP: estimated 6843871 tape blocks.
  DUMP: Cannot open output "/mnt/hd/FBSD/".
  DUMP: Do you want to retry the open?: ("yes" or "no")

i tried to reopen the output but it had no effect.

P.S.:ad0s3a is my root directory (/)
s3b is /home and
s3d is my swap space