if you won't include -a flat, dmup will calculate some tabe blocks or something, and write only as much as these can be filles (whic is very small), and then ask to dumb question... i don't remember what exactly....
Code:
-a ``auto-size''. Bypass all tape length considerations, and
enforce writing until an end-of-media indication is returned.
This fits best for most modern tape drives. Use of this option
is particularly recommended when appending to an existing tape,
or using a tape drive with hardware compression (where you can
never be sure about the compression ratio).
Whit -a dump will write as long as there is space on disk. or dump is ended.....
and there won't be problems.....
I did it many times....
to restore dump you need to format slice (newfs /dev/slice or newfs -U /dev/slice), then mount it, ch into mount point, and restore -rf /path/to/dumpfile
or bzcat /pat/to/dumpfile.bz2 | restore -rf -