Substituting DSHD for DSDD disks (or DS2D if you prefer)

[Chuck Guzis]

On 10/24/2015 06:59 PM, Eric Christopherson wrote:
> I know Chuck Guzis has written about this, but I don't see that he's done
> so publicly in the last few years, so I thought I'd ask here about his and
> others' views on the perennial question of whether (some) 3.5" DSHD disks
> can be reliably used in DSDD-only drives. The oft-repeated claim is that
> writing can appear to work just fine, but that even a few months later read
> errors will occur.

My opinions on Herb's retrotechnology site still hold--with one addition.

You can sometimes get 3.5" HD disks that have been used, but now refuse 
to accept a format by first performing a DC erase.  That is, get a very 
strong rare-earth magnet, and moving in a helical path (i.e. circular, 
starting close to the disc, slowing moving away), perform an erase pass. 
Following with an AC erase can sometime inject new life into the disk. 
I've tried this several times and it does seem to work.

As Fred mentioned, the "write them now, but not read them later" case 
applies to HD 5.25" disks written in DD mode.  But even that's not a 
sure thing.  I've handled batches of DD-written 3M HD floppies that were 
more than 20 years old and they read fine.

I've never tried DD writing onto ED floppies.  I suspect that it won't 
work at all.  The coating characteristics are just too different.

--Chuck