The replies:
gibian@stars1.hanscom.af.mil (Marc S. Gibian) said not to mess with
cpio. Use a bulletproof, tested backup product such as Solstice Backup.
This is good advice and we are switching to Solstice Backup as soon as
we can install it. The only catch I know is that the automatic tape
switchover software is sold separately from the Solstice Backup base
product.
A few people suggested using ufsdump instead of cpio. Good idea in
general but not for us. We can't take the file system offline.
Tim Evans <tkevans@eplrx7.es.dupont.com> suggested writing an "expect"
script to do what I wanted. He's probably right that "expect" is a
good tool for the job.
Now the punchline. After I posted the question I learned the real
problem. It turns out the tape had plenty of capacity. The tape drive
needed cleaning! I check for warning messages daily in
/var/adm/messages but none had appeared for days so I thought I was
safe.
The symptom was that the tape drive went offline and could not be
brought back online. If this happens to you, consider cleaning the
drive even if the system messages don't say cleaning is needed.
My thanks to all respondents.
-- Jim Stern -- Views here are my own, not Northrop Grumman's. (El Segundo, CA)