ufsdump writes a sequence of fixed-size records. When ufsdump receives notification that a record was only partially written, it assumes that it has reached the physical end of the media. This method works for most devices. If a device is not able to notify ufsdump that only a partial record has been written, a media error occurs as ufsdump tries to write.
DAT devices and 8mm tape devices detect end-of-media. Cartridge tape devices and 1/2-inch tape devices do not detect end-of-media.