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From: Magill@CIS.UPENN.EDU (CETS Operations Manager)
Newsgroups: mod.computers.vax
Subject: multi density tape drives and backup
Message-ID: <8608181919.AA03459@ucbvax.Berkeley.EDU>
Date: Sun, 17-Aug-86 13:00:00 EDT
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Posted: Sun Aug 17 13:00:00 1986
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Unless DEC has stupid tape drives....
A dual density tape drive will READ the tape at the correct density indepentdly
of whatever density you tell it. (Assuming of course that your tape is written
at a compatible density).
(Also assuming that you have a drive which allows software selection of density,
and does not rquire a manual switch setting.)
In theory, it should not be possible to write multiple tape densities to the
same tape. I know this is true for software selectable tapes, and for the few
manually selectable drives I have seen. 
I have seen the same problem around here ... once several months ago. And I
don't remember what we determined was the cause... as for the tape, backup
simply could not read it, therefore it was unrecoverable unless someone
wished to write a program to read it.
FLAME ON:
I really wish DEC would support standard labels under BACKUP, then many of
this type of problem would vanish. Any tape MOUNTED/FOREIGN is not using
standard label processing. Backup does not write standard labels.