SUMMARY: dd fskip=1. getting info from corrupt tape.

[Herb Peyerl]

A couple of weeks ago I posted a question asking about whether it
was possible to read past an end-of-file marker on a 6157 tape... I
got a couple of responses which follow this short prelude... The
actual situation is not yet resolved... What actually happened was
as follows:

The company I used to work for has clients with RT's and they write
software for these clients... They support the software via modem
and it was during just such a support call that a programmer accidentally
typed "rm database" instead of "mv database database.bak"... She then
proceeded to copy a bunch of other stuff around before realizing her
mistake... This database file (Patient information) was on the order
of  50-60 meg... Later they decided to go to the backup but they 
then realized that when the backup was being done, it would reach the
end of the tape and see "Insert volume 2 on /dev/rmt0" or something
to that affect... The receptionist doing the backup didn't know what
this meant so just hit return, consequently finishing the backup on
the first tape... It only wrote about 22 meg over top of the 150 that
was written previously and they're sure about the database file being
after the 22 meg... However, there was nothing to be done about it.. We
managed to get back about 20 records before getting nothing but errors
off the tape...

Anyways, here are the responses I received... Thanks guys...

--------------------------------
QIC tape format requires an erased piece of tape of 50" after the
last end of file, and if you are on track 0, it erased the entire
width of the tape - all tracks - for 50 inches.  Not likely you
can read much without huge contortions.
A. Lester Buck		buck at siswat.lonestar.org  ...!texbell!moray!siswat!buck
---------------------------------
These cartridge tape units write 9 tracks on the tape, one at a time.
When track 1 is full, the drive positions the read/write head on the
next track and continues on track 2 in reverse direction, etc.
These tape units also have an erase head. This head erases ALL NINE
TRACKS when track 1 is being written to. I assume that the shorter
file that has overwritten some of the tape has hit the end of the
first track. This means that there is only one file left on the tape.
Sorry, you're out of luck, and it is not IBM's fault; all these tape
drives work this way.
Now if the second file was very short (like your 1MB example), there
would still be something left of the second file, but it may have
up to 4 gaps of about 2 MB in it.

My condolances & best wishes for xmas, the new year and your file
(just in case I was wrong).

Greetings from Delft, The Netherlands!
P. Knoppers, Delft Univ. of Technology, The Netherlands, knop at duteca.tudelft.nl
--------------------------------------
-- 
---------------------------------------------------------------------
UUCP: herb at blender.UUCP   ||  ...calgary!xenlink!blender!{herb||root}
ICBM: 51 03 N / 114 05 W  || Apollo Sys_admin, Novatel Communications
"The other day, I...... No wait..... That wasn't me!" <Steven Wright>