Multiple archives on one tape

[kcables at RELAY.NSWC.NAVY.MIL]

	Hi everyone.  I'm trying to write a script to do incremental
backups on an 8mm cartridge.  What I'd like to do is create multiple
archives on the same volume, i.e., start at the beginning of the
cartridge for Monday's archive, append Tuesday's archive to Monday's, etc.
The man page for tar says that the -r (no rewind) option only works for the 
9 track tape, so I'm assuming I can't use tar for this purpose with an
8mm cartridge, or even if there is a way to use tar the SA book says
it's unreliable.  The System Admin class book, in the chapter "Backups", 
says that multiple archives are possible, and they recommend using bru 
(p. 15).  Unfortunately, they don't explain the semantics.  So
then I tried bru, whose man page doesn't say anything about multiple 
archives, only multiple volumes.  With bru you must specify one of the 
mode options, and the only one that instructs bru to actually write 
to the tape is -c ("create a new archive").  According to the man page,
-c causes the writing to start at the first block, which I'm not
sure means at the very first block of the cartridge, or at the 
beginning of the next full block (i.e., it skips any remainder of the
current block).

'tape8' and 'tape8nr' are symbolic links to the cartridge tape drive.

This is what I tried:

	find <files to save> | bru -cvd -f /dev/tape8 -

which works fine for the first archive.

Then I tried:

	find <other files to save> | bru -cvd -f /dev/tape8nr -

and it responded with

	bru: warning - assuming end of volume 1 (unknown size)
	bru: load volume 2 and enter device [default: /dev/tape8nr] >>

What am I missing here?    Thanks in advance. 

My address is kcables at relay.nswc.navy.mil