Other than really ancient DOS versions (1.x mostly) IBM PC floppies are either DD (360K) or HD (1.2M). FZ-506 is, according to manual, a HD drive so it will read both formats but it can't write DD floppies properly. Such floppies would be readable only on HD drives. That being said if you do not intend to (re)create 360K floppies it's of little concern to you, reading both formats is what you want.

Your confusion comes from the fact the manual gives unformatted track/disc capacity but if you look into the Specification and the table there you'll see 15-sector of 512 bytes format for HD mode, which is 1229 KB = 1.2M
The manual doesn't offer a PC-compatible example of DD format.

It's a normal 5.25" High Density drive.

With the standard IBM PC AT format - 1.2 MB.

Other computers - or non-standard software on the PC - may use different formats.
Therefore, the drive's manufacturer only specifies the "unformatted" capacity - 1.6 MB.

Wow. Thanks so much for the detailed explanation guys. I'll give it a try then

Wow, I got the drive, cleaned up a little, and it works! I'm also amazed that like 30 floppies I've been having at home for like 30 years already are still working, and just like 4 have 1 bad sector.

Now I'm puzzled whether these "double density" floppies I have are all (formatted?) 360Kb. Was there any special label for 1.2Mb disks back then to differentiate? Possibly this batch comes from 286 era, could it be the original drive used was 360Kb only (hence they got formatted to that size)?

fix_metal wrote on 2025-03-27, 22:12:

Now I'm puzzled whether these "double density" floppies I have are all (formatted?) 360Kb.

If they are 5.25" Double Density, and were used on a PC, the 360 KB format was pretty much the only possibility.

Was there any special label for 1.2Mb disks back then to differentiate?

They should be labeled "High Density".

Possibly this batch comes from 286 era, could it be the original drive used was 360Kb only (hence they got formatted to that size)?

More likely XT, rather than 286.
XT machines were by default shipped with 360 KB drives, AT (286) machines had 1.2 MB ones.

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)

All the floppies I have here are Double Density Dual Side 48TPI.
Long story short; 360Kb is the right fit.

I'm looking at the data, and I spotted a copy of this IBM Disk Writer 1.5 software. Alongside several text files written by my uncle. The funny side of the story i even though ScanDisk is not finding any bad sector most of the text data is corrupted anyway, like bad characters. Overall I can read it, but I see patterns somehow, like any year last digit is corrupted. I'm wondering if this is some kind of ""encryption"" from the 80s - I'd say more like encoding, that is.

A HD drive should not have problems reading DD disks (as long as it's configured correctly as a HD
drive in BIOS, and you are using a controller capable of HD).

If it were actual bad sectors, you should get read errors, as sector CRC would fail.
This suggests that the data as "as it was written".

Do you know what software was used to create these files?

If it was some proprietary document package, it might use non-standard characters.

Another thing to check - look at the files with a binary/hex viewer and see if the high-bit
is set in the corrupted characters - some text systems "back in the day" used the
(normally unused in ASCII) high bit to mean something "special".

(which was sometimes as simple as "we've saving the file space that would have
been used by some common character(s) (like Space or Newline) which would have
followed this one by setting it's high bit)

You mention "IBM Disk Writer" - is this some sort of text entry tool?
Perhaps you can run it and open the files and see if they look more normal.
(obviously this should be done on copies)

fix_metal wrote on 2025-03-27, 22:12:

... Was there any special label for 1.2Mb disks back then to differentiate?

Not always .. if you are lucky a printed label will indicate Double or High density.

Another good clue - almost all DD diskettes have a visible re-enforcing rung around the center
hole ... most HD diskettes didn't have this!
(yeah, I know - late response - just re-reading the thread)

DaveDDS wrote on 2025-03-28, 21:34:

A HD drive should not have problems reading DD disks (as long as it's configured correctly as a HD drive in BIOS, and you are us […]
Show full quote

A HD drive should not have problems reading DD disks (as long as it's configured correctly as a HD
drive in BIOS, and you are using a controller capable of HD).

If it were actual bad sectors, you should get read errors, as sector CRC would fail.
This suggests that the data as "as it was written".

Do you know what software was used to create these files?

If it was some proprietary document package, it might use non-standard characters.

Another thing to check - look at the files with a binary/hex viewer and see if the high-bit
is set in the corrupted characters - some text systems "back in the day" used the
(normally unused in ASCII) high bit to mean something "special".

(which was sometimes as simple as "we've saving the file space that would have
been used by some common character(s) (like Space or Newline) which would have
followed this one by setting it's high bit)

You mention "IBM Disk Writer" - is this some sort of text entry tool?
Perhaps you can run it and open the files and see if they look more normal.
(obviously this should be done on copies)

Tried this IBM Disk Writer software, it's rather odd the internet has no trace of it. But no, opening files with that is just pretty much the same as opening with "edit". I'm pretty sure back then there were hardware "licences" on stuff to be attached to parallel port. I've got some myself from whatever backup software my dad used in his office in early 90s. It shouldn't be rocket science to fix these files to readable, I'll have a look.