[rescue] mt erase

brian wheeler bdwheele at indiana.edu
Thu May 10 17:31:11 CDT 2007

On Thu, 2007-05-10 at 16:59 -0500, Doug McLaren wrote:
> On Thu, May 10, 2007 at 01:31:15PM -0400, Lord Doomicus wrote:
> | I've erased tapes using  dd if=/dev/random of=/dev/mt0 bs=<block size  
> | of tape> count=<number of blocks on tape>
> | 
> | if you don't have /dev/random ( not on a lot of older platforms ),  
> | you should be able to use /dev/zero.
> Depending on your OS, you might prefer /dev/urandom.  At least under
> Linux, /dev/random blocks when it runs out of entropy, and so to come
> up with many gigabytes to erase a tape would take ages.  A quick test
> on my computer showed /dev/random emitting data at about 16 bytes/s --
> so that 5 GB tape will take about 10 years to erase.  Perhaps you can
> speed this up signifigantly, but ...
> /dev/urandom will reuse it's entropy once it runs out, so it's go MUCH
> faster.  (And the data will probably be more than random enough.)
> Using /dev/zero, especially just once, will certainly make reading the
> tape much harder, but I wouldn't rely on it to protect sensitive data
> from anybody who has physical access to the tape.
> (I'm not really looking to get into secure deletion here ...)
> Do be careful if you decide to use a degausser -- you need a really
> strong one for modern tapes.  For example, a cheap degausser found at
> Radio Shack for cassette tapes will have zero effect on a DLT tape, no
> matter how long it's applied -- the tape will still be readable
> without even any errors.

Not to mention that if you do use a strong degausser you may ruin some
types of media that have a servo track on them (IBM 3590?  Others?)


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