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From: randy@AMSAA.ARPA
Newsgroups: mod.computers.vax
Subject: Re:  BACKUP tape problem
Message-ID: <8608182002.AA04047@ucbvax.Berkeley.EDU>
Date: Sun, 17-Aug-86 18:04:39 EDT
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Posted: Sun Aug 17 18:04:39 1986
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>>    We have a rather serious problem here that I hope someone will be able to
>>    help us with: we've just discovered that many of our weekly save tapes are
>>    unreadable by Backup, although Backup wrote them just fine.

>which you're writing the tape.  Given this fact, it seems possible that
>what's happened to you is that the tape drive was set to allow software-
>selectable densities, no /density qualifier was used in the mount command,
>a /density qualifier was used in the BACKUP command, and the density specified
>was not the drive's default density.  In this case the first tape would be
>written at the density selected in the backup command, and all subsequent
>volumes would be written at the drives default density.  If, whe you 
>tried to read the tapes, software-selection  if density were disabled, only
>the first tape would be readable.

     All tape drives I have ever seen that work with VMS(or any VAX
running any operating system) will READ a tape at the proper density, no
matter what their default density is, and no matter how they are
mounted.  Put a tape on your drive and set it manually(or through
software) at the improper density and try to read it.  You will notice
that the tape always will jiggle back and forth a few times when
starting a read to see if it has the proper density, and when it finds
what it thinks is the right one, it will change the density - watch the
lights.

Randy