198. High-density floppy discs
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From time to time, you may come across 5.25" discs referred to as "High
Density". After "Single", "Double" and "Quad" density, you may be fooled into
thinking that "High" density must be a good thing. Think again!

Single, and double density discs, whether single or double-sided, single or
double-density, 40 or 80-Track, tend to be made on much the same production
lines. After testing, the best discs can be certified as 80T (96 tpi)
double-sided double-density, and lesser ones as perhaps single-sided, or
single-density, and so on. Obviously, manufacturers cannot conveniently
arrange exactly the right failure rates to achieve the exact mix of quality
of discs required to satisfy the demand, so quite often discs are of better
quality than officially certified.

Quad density discs tend to be made on separate production lines, and are of
higher quality. Nonetheless, they are basically made of the same materials,
and it would do no harm to use them if they were cheaply available.

High-density discs, on the other hand, are a very different thing altogether.
They are intended for systems which pack 1.2 Mbytes onto one disc, (usually
superior IBM compatibles), and the magnetic material is different. A crude
comparison might be that between ordinary ferric audio cassettes and chrome
cassettes. A much stronger magnetic field is required to magnetise the
material properly, and only drives intended to do this are suitable. They
sense whether the disc is 'ordinary' or high-density, via an extra notch, and
alter the head current accordingly.

What this boils down to in practice, is that High Density discs can be very
unreliable and unpredictable when used with 'ordinary' drives such as one
has connected to a BBC Micro. Frequently, they do not even format properly,
or if they do, funny things happen later. In other words, DO NOT USE THEM!!

The situation with 3.5" discs is similar, though apparently many modern
drives normally used in a double-density mode will happily cope with
high-density discs. This applies to later Arcs, though I am not sure about
the earlier 440s amd 300 series. However since High Density discs are VERY
expensive, there seems little point in finding out!