Date   : Mon, 22 Mar 2010 00:20:44 +0000
From   : pete@... (Pete Turnbull)
Subject: Re-using floppies

On 21/03/2010 20:45, Alex Taylor wrote:
> On 21 March 2010 06:58, Mike Tomlinson <mike@...> wrote:
> 
>> But don't buy HD ones.  Even if they seem to work, they are the wrong
>> spec and you will lose data.
> 
> I've read somewhere that this a combination of the issues of magnetic
> coercivity and the fact that modern HD floppies come preformatted as
> FAT12, so so reformatting them with a double-density format doesn't
> work properly. The same thing I was reading went on to say that the
> disks are close enough in spec to work if you erase them in a bulk
> eraser first, then use them as DD disks. I have no idea if this works
> though, not having access to a bulk eraser.

If you're talking about 5.25" disks, the difference between HD and SD/DD 
disks is significant, but not as great as the difference between 3.5" HD 
and DD. So data written on the "wrong" type of 5.25" floppy is 
unreliable over time, but may appear to work short-term.  Don't trust it.

However, the difference in coercivity between 3.5" HD and 3.5" DD is a 
factor of two, and it is very unreliable.  Don't even think about it; 
Mike is right.

The format is irrelevant; when you (re)format a disk it gets completely 
overwritten -- or not, depending on the drive's write current, but not 
on the previous format.  Using a bulk eraser or not won't make any 
difference to the reliability.

-- 
Pete                                           Peter Turnbull
                                               Network Manager
                                               University of York