[wellylug] Magnetic tape question

[Ewen McNeill]

In message <200312171355.20962.rjhinde at mail.computers.org.nz>, Robin Hinde write
s:
>> I have a number of DLT IV tapes that have been used in an older DLT tape
>> drive, and now cannot be used in a DLT I.  [Suggestion is to bulk 
>> erase them so drive will accept them again; DLTtape(tm)IV media is
>> rated at 1850 Oersteds -- need three times to be sure.]
>
>I'd say you are SOL:  [....]
>"[....] Tapes of over 750 Oe coercivity are referred to as Class III, with 
>no known degaussers capable of fully erasing them being known [19], [...]"
>
>The above from Peter Gutman's article at
>http://www.cs.auckland.ac.nz/~pgut001/pubs/secure_del.html

FWIW, Peter Gutman is looking at it from the point of view of "what do
we need to be REALLY sure that nothing can be recovered from this tape"
(ie, a security point of view -- that's what he did his PhD in).  

The original questioner probably just needs to change the content on the
tape enough that it's "random" on a casual read as far as the DLT I tape
drive is concerned, so it doesn't start making the wrong assumptions
about the tape.

Any moderately strong bulk eraser will probably have enough effect on
the tapes to make the old data not read off reliably with a standard
tape drive -- and hence to make a DLT I willing to use them again.

Perhaps even some other strong magnetic fields -- such as the trolley
bus motors.  Although I'd have thought there was too much metal in the
way in that case.

Ewen