It should be no problem. I've heard of issues where a previously formatted HD disk had problems when later formatted in a LD drive, but I think unformatted disks should work fine.

Yeah, just to play around formatting an HD floppy as DD in a DD drive works just fine.
In theory data integrity/security/duration/still searching for the correct word in English will be worse that using a proper DD disk, but as I understand you just want to create some floppies and use the machine. Go ahead.

Excellent purchase btw 😉

Not sure whether by low-density you mean single-density or double-density, but I'm guessing DD.

To expand on what konc said, the magnetic coating on the disk is slightly different, so the DD drive will have a harder time getting the data to stick on a HD disk - the signal will be weaker.

I've heard the result from this is that the data will become unreadable sooner, but this seems to mean "after 10 years" instead of "after 20 years" so it probably won't be a problem for you. And as long as you have a copy of the data, you can just reformat the disk and copy it back again should you ever run into problems.

The real problem is switching a disk between DD and HD drives. DD drives have 40 tracks, while HD drives have 80 tracks, with each being half the width so it can fit twice as many into the same physical space. This means when a HD drive accesses DD media, it skips every second track so that the 80-track drive mechanism behaves as if there are only 40 tracks. The HD head is of course half the width of the DD head, but it has no problem picking up the DD track even though it's only reading one edge of it.

The problem is if you write data with the HD drive, the head is half the width so you are only writing on the edge of the DD track. If the disk is completely blank (degaussed, unformatted) then this is ok - the DD head will pick up the data from the edge of the track with no other interference. But if there is already data on the disk, then the HD head can only overwrite the edge of the track, leaving the rest of the track untouched, with the original data intact. The DD drive will then see the original track, with different data along one edge, and its wider read head will combine the two signals into one that makes no sense at all, resulting in read errors.

So once a disk has been written in a DD drive, you can no longer reliably write to it in a HD drive because you will never be able to completely update the full-width magnetic signal on the disk surface. Likewise if the HD disk has previously had all 80 tracks written to (by being formatted as 1.2MB) then when you reformat it as 360k in the HD drive, you'll only be writing to the edge of the DD tracks, so the DD drive will see the new and original data at the same time again, causing read errors.

If you format the disks in the DD drive then they will work fine, as the DD head can completely overwrite the full-width tracks. But if you then try to copy data onto the newly formatted disk in the HD drive, you'll be back to the original problem where the data gets written to the edge of the track, so the DD drive will see half the track being formatted and the other half having data on it, and the combined signal will look like nonsense.

So long story short, you can use HD disks in a DD drive, but only in two specific cases. 1) the disk is formatted in the DD drive and only ever used in DD drives, and 2) you use a degaussing wand to completely erase the disk, then use the HD drive to format it in DD mode and write data in the HD drive. You can keep using this disk as long as all writes are done in a HD drive - once you write on the DD drive the disk becomes DD-only and you need to degauss it again before you can write to it in a HD drive.

Sorry if this sounds a bit confusing, it ended up being a bit of a long-winded explanation so I hope it makes sense!

Malvineous wrote:

To expand on what konc said, the magnetic coating on the disk is slightly different, so the DD drive will have a harder time getting the data to stick on a HD disk - the signal will be weaker.

I'd never heard of this, so I looked into it further and it seems it is a thing. More info here: http://www.retrotechnology.com/herbs_stuff/guzis.html

Malvineous wrote:

The real problem is switching a disk between DD and HD drives. DD drives have 40 tracks, while HD drives have 80 tracks

All useful info, but the OP is asking about 3.5 formats, which have 80 tracks for both DD and HD. The 40/80 thing only applies to 5.25" PC formats.

Oh whoops, the "low-density" got me thinking 5.25", didn't notice the mention of 3.5" 😵

Malvineous wrote:

Oh whoops, the "low-density" got me thinking 5.25", didn't notice the mention of 3.5" 😵

Yes, the track width is the same for DD and HD 3.5" drives, its just that HD drives have double the sectors per track.

A DD 3.5" drive/disk has a coercivity rating of 600 oersted while an HD drive/disk has a rating of 700. Coercivity is the amount of power to force a flux transition to a magnetic coated disk. The higher the number, the stronger the signal. Because HD disks are more densely packed than DD disks in the same amount of space, a higher coercivity rating is required to write to them. Since the ratings are not too far apart, writing to a HD disk in a DD drive should work, but I would not trust it for anything critical.

A DD 5.25" drive/disk has a coercivity rating of 300 oersted while an HD 5.25" drive/disk has a rating of 600. (I'm not sure if there were SD 5.25" disks or drives, that may a distinction only valid for 8" disks and drives). Writing a HD disk in a DD drive is not reliable. Reading a DD disk in an HD drive is possible even though the HD drive's head is only half as wide as a DD drive's head (here is where the 40 vs 80 tracks come into play). Writing to a DD disk in an HD drive is possible, but do not expect to use that disk in a DD drive unless you degauss it.