This informative message was returned when testing a windows sbs 2011 backup to a Buffalo Nas share. The error relates to the format of the backup not being compatible with the file system format on the NAS. Not that you could easily tell that from the error of course.
In terms of compatibility it is surprising to find that for example plugging an ntfs formatted external disk on to the buffalo NAS gives you a read only share – despite allowing you to set the rights to read/write. Also that the XFS format is not natively understood by Windows depite having been about for quite some time.
So its back to basics to figure out a reliable backup mechanism for the client – given the nature of the hardware and filesystems in play in this case.