Al poner -f tienes que indicar un fichero, no un directorio. Por ejemplo:
xfsdump -f /mnt/usbdisk/xfs_test/fich_img_de_srv.extension /srv/samba
Pues ahora obtengo esto:
horus:~ # xfsdump -f /mnt/usbdisk/xfs_test/imagen_samba.xfs /srv/samba
xfsdump: using file dump (drive_simple) strategy
xfsdump: version 2.2.30 (dump format 3.0) - Running single-threaded
============================= dump label dialog
==============================
please enter label for this dump session (timeout in 300 sec)
-> test
session label entered: "test"
--------------------------------- end dialog
---------------------------------
xfsdump: ERROR: /srv/samba does not identify a file system
xfsdump: usage: xfsdump [ -a (dump DMF dualstate files as offline) ]
[ -b <blocksize> ]
[ -c <media change alert program> ]
[ -d <dump media file size> ]
[ -e (allow files to be excluded) ]
[ -f <destination> ... ]
[ -h (help) ]
[ -l <level> ]
[ -m (force usage of minimal rmt) ]
[ -o (overwrite tape) ]
[ -p <seconds between progress reports> ]
[ -q <use QIC tape settings> ]
[ -s <subtree> ... ]
[ -v