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

Eric Christopherson echristopherson at
Sat Oct 24 23:06:04 CDT 2015

On Sat, Oct 24, 2015 at 9:14 PM, Fred Cisin <cisin at> wrote:

> On Sat, 24 Oct 2015, 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.
> That was certainly the case with 5.25", but THAT was a difference between
> 300 Oersted and 600 Oersted.  WAY OFF.
> But, with 3.5" disks, the difference is between 600 Oersted and 720?
> Oersted.  THAT is close enough.
> For BEST results, I think that it would be better to use the right ones,
> but unlike 5.25" disks, with 3.5", you can get away with it.
> Elsewhere on the page (I don't recall now if it was Herb or Chuck that said
>> it) it was conjectured that HD disks that have never been formatted as HD,
>> -OR- disks that have gone through a good degaussing, will have better luck
>> retaining data. What does everyone think about this? And would an
>> electromagnetic library security system (the kind that's like a tube
>> through which checked-out materials are put; often with a caution not to
>> put tapes or floppies through it) be a suitable degausser?
> Probably a very good idea.

OK. I just wanted to ask, in case running a floppy through that contraption
would actually mess up its magnetization so badly that it couldn't then be

> Some Windoze machines will check for existing format before formatting,
> and be somewhat uncooperative about reformatting as a different density.
> The one time that it is critically important to bulk-erase or use virgin
> disks is when writing 48tpi disks in a 96tpi drive.  When a 96tpi drive
> RE-writes a 48tpi disk, as 48tpi, it can not clear the edges of the track
> completely.
> Are we really running short of "720K" floppies?
> I thought that AOHell had sent out enough snail spam with disks to supply
> us forever!

I had to laugh at that. Another list member recently told me that AOL disks
are the ones he's had the most success with recently. I don't know how many
of them were 720KB, though. In any case, I think I only started getting AOL
dis(c|k)s in the CD-ROM era, unfortunately.

But anyway, it does look like DD disks are more expensive; that, coupled
with the fact that a lot of HD disks in the wild are going to be newer,
makes me want to buy some HD ones instead. But that second part might be
more of a bad thing, if it's true that floppy QA went downhill later on.

On Sat, Oct 24, 2015 at 10:48 PM, Chuck Guzis <cclist at> wrote:

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

Thanks for that.

> 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.

Fascinating -- I didn't know there were AC and DC magnetic fields. How
strong is "very strong", and would the library device I mentioned count
toward "an AC erase"? Should I assume that just doing an AC erase would be

        Eric Christopherson

More information about the cctalk mailing list