5.25" floppy drives come in a few "flavors", with different combinations
or a few defining characteristics:
1. Number of sides - one or two.
2. Number of tracks, this will be either 40 or 80 (technically the original
5.25" drive - the SA-400 was 35 tracks - but those are very uncommon and
were never used in a PC)
2. Single/Double or high density. Single/Double was physically the same drive,
the different was the controller these drives rotate at 300rpm, and have a
data transfer rate of 250kbps. High density drives rotate at 360rpm and have
a data transfer rate of 500kbps.
The common PC drives were either:
40 track double sided, double density (360k)
-or-
80 track double sided, high density (1.2m)
Single-sided and 80-track double density drives do exist, but they were not
commonly used on a PC.
Telling the drives apart:
Single-sided is fairly obvious, they have only a pressure pad at the top head
position.
40 or 80 tracks can often be determined by the stepper mechanism, but be
aware that there are drives when the stepper "steps" more than once for one
track - best way to hook it up, run my ImageDisk which has a Test/Align
function which will let you precisely control/step the drive, recal to
track-0 then step to 40 - on a 40 track drive the head will move to near
the inside.. on an 80 track drive it will step only 1/2 way.
DD or HD is also tricky, It you have a scope you can measure the pulse rate
coming from the index sensor to determine 300 or 360 rpm. ImageDisk also has
a "test rpm" function which can do this for you.
I know these are not something you can tell from a "bay" listing...
I've found that DD drives more often have a DIP termination resistor
pack in a socket so it can be removed (only supposed to be installed
on the last drive in the chain). HD drives more often have a jumper.
(but there are variations of this in different manufacturers)
Another common "visual" HD drive are almost always white or cream colored
bezels. DD drives are much more common with black bezels - but white ones
do exist. If you can see the activity LED in the photo well enough to guess
it's color - HD are almost always GREEN, DD more often RED (again different
variations with different manufacturers)
Btw1 - the "issue" with HD drives not writing DD properly is because it has a
thinner head (80 . vs 40) - when writing a disk that was previously written
on a DD (40 track drive with thicker heads) - some portion of the original
edges will remain - which can confuse 40 track drives as their thicker head will
see both signal portions - not a problem when reading on an 80 track drive, or
if the disk was un-formatted/bulk-erased first (no wider data component).
Btw2 - HD drives need a controller that supports 500kbps - this came with the
AT - original PC/XT didn't support HD drives (unless used a different controller)