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From: raj@pollux.geog.ucsb.edu (Richard A Johnson)
Newsgroups: comp.unix.aix
Subject: HELP!  Bad 8mm tape!
Keywords: 8mm, tape, backup, restore
Message-ID: <11579@hub.ucsb.edu>
Date: 25 May 91 03:44:12 GMT
Sender: news@hub.ucsb.edu
Distribution: comp
Lines: 54

HELP!  I need help from anyone out there who has dealt with 8mm video tape
drives a lot!  Here's the problem:

I backed up this person's files on 8mm video tape (just the /u files that is).
Then I totally reinstalled their system from scratch bring it up to the latest
rev. later (3005 - but it's not really relevant).  I then proceeded to restore
their files.  That's when I found that the tape had a bad spot!  If you
manually advance the tape a little you find a place where it's crinkled a
little bit.  When the drive gets to that part it stops reading and returns
an error ("The media surface is damaged").

Eventhough "restore" says it's "Ignoring data and continuing", it doesn't get
anywhere because the drive simply can't track past that bad part!

I have tried the obvious:
1) tctl -f/dev/rmt0.1 fsr somenumber
	(where "somenumber" has been anything from 1 to 500)
	It doesn't skip past the bad point
2) tctl -f/dev/rmt0.1 fsf somenumber
	Still doesn't skip past the bad part.

I could manually forward the tape past the bad part but when you put it into
the drive the first thing the drive does is rewind it!  I can't find any
IOCTL, etc. which will tell the drive not to rewind the tape.  Maybe there's
a dip switch on the drive itself?

The ONLY thing I can think of would be to take the tape apart, cut off the bad
part at the beginning, reattach the rest to the tapeup spool, and use dd to
read the data straight into a file.  Then write a program to read the
restore file (IBM says there isn't any documentation on the format of the
file!) and get as much as possible back.  I'm not completely sure this would
work since I bet the drive writes checksum information onto the tape and when
you try to get it to start in the middle of it, it's not going to like it
at all. 

The reason why I figure the drive writes extra info on the tape is that
otherwise how would it know that the "media surface is damaged"?  If this
isn't how it tells, then there may be hope for flattening the tape out?
I don't have high hopes for that simply because while I've been writting this
I was trying to read some of my incremental saves.  One tape in particular
read just fine the first 4 or so times and then started giving the same type
of problem!  Now I doubt that the tape got mangled while it was still in the
drive!  (Yea, I know, maybe it did.  That would mean I have a bad drive and
all bets are off until I get it fixed.  I don't think that's that big of a
chance really.)

Am I missing anything?  Is there some way out of this?  How do you read past
a bad part of an 8mm tape?  It seems impossible!  You can bet I'm going to
re-read ALL saves to 8mm tape in the future!  (I know, I should have
this time...)

Many thanks for ANY help or ideas!

/raj