Formatting DSHD 5.25" diskettes as DSDD

[Twylo]

I've been looking through Google and the forum search trying to find a good summary answer to a very basic question. It's just been so long since I commonly used 5.25" diskettes that I've outright forgotten almost everything I once knew.

I've been getting my new (to me!) IBM 5150 up and running, and I don't have any DSDD diskettes. I do have a large stack of DSHD diskettes, so I formatted several of them DSDD (in a 1.2MB drive) and wrote DOS 2.10 and 3.3 images to them.

They... sort of work, mostly, but they're extremely flakey, with lots of read errors. I think this is to be expected; I vaguely remember that HD diskettes just can not reliably be formatted as DD. Is this correct?

Would someone mind giving a quick summary of how double density differs from high density, and why the media is not backward compatible? I know I'd appreciate the refresher.

[In the meantime, I found some DSDD diskettes on eBay and they're on their way to me!]

 

[Bungo Pony]

I remember trying to format a DS HD floppy on my C-64. It wasn't too happy.

I'm pretty sure that HD floppies are made from a different compound than the DD floppies are. Kinda like Normal and Cr02 cassettes.

As for finding DD floppies, I picked up a few hundred from my local freecycle group. I've got tons to keep me occupied for years!

 

[Chuck(G)]

HD 5.25" media differs from 2D substantially in two ways.

The most important is the coercivity of the magnetic material used--the magnetic field strength needed to magnetize a material.

The coercivity of 2D media is 290 oersteds and about 560 for HD. Thus a much stronger write current is needed for HD.

Another aspect is the coating thickness--about 2.5 microns for 2D and about half that for HD. This means that what you manage to write to HD media with a 2D drive will read back with a lower signal level.

The third factor is magnetic particle size--generally smaller for high-density media. Not particularly important, except to high-density recordings.

The fourth factor is loading--the ratio of magnetic material to binder. Generally, HD media has a lower ratio to avoid problems with "bit crowding" on the inner tracks, where a magnetization change can actually push one following further away. (this is the reason for write precompensation circuitry).

In short, don't do it. You won't like it.

 

[Twylo]

HD 5.25" media differs from 2D substantially in two ways.
...

Fantastic, this is exactly the explanation I was looking for. Thank you for taking the time to explain it so clearly!

My DSDD diskettes arrived in the mail today, still shrinkwrapped and in excellent condition. I wrote a set of DOS 3.30a STARTUP/OPERATING images to them, and they have no issues reading or writing, so I'm back in business.

 

[Lutiana]

Want some more DSDD disks? I have a large cash of 'em and I can get them too you for a little more than shipping.