How to deal with bad sectors on 800K Mac floppies

All questions regarding the dumping of media go here.
Post Reply
Alex
Posts: 14
Joined: Wed May 26, 2021 11:44 pm

How to deal with bad sectors on 800K Mac floppies

Post by Alex »

I'm trying to image two 800K Macintosh floppies from 1994-95. These contains my high school programming/word processing projects which I created with a Macintosh Plus at the time.

I've tried the DTC GUI under Windows with the following floppy drives:
  • Sony MPF 920
  • Samsung SFD-321B
  • Teac FD-235HF (I have 3 of these)
Both floppies (on all of my drives) have bad sectors (not always on the same sector). I tried Density Line at Low and High and with 15 Retires instead of 5 but I'm still unable to get a 100% image (at least in all of my tries I managed to get a readable IMG file of both floppies - with some corrupt files).

I'm waiting on 2 new floppy drives to arrive (a Panasonic JU-257A827P and a NEC FD1231H) but I doubt it will work. My final step would be to clean the floppies (with this technique) and retry my dumps.

Does anyone have suggestions for me? I can read (I think) all the floppy across multiple drives, can I combine multiple images into one?

Anyway, thanks for the help!
User avatar
mr.vince
Posts: 2198
Joined: Tue Oct 05, 2010 5:48 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by mr.vince »

Yes, you can do raw stream dumps with the corresponding Mac format as a guide, so it will retry on bad reads. You can then dump all disks on all drives. By looking at the logs (you can pipe the output with ">") you can then copy all good stream files to one folder and then re-use this stream as input an basically "replay" the image generation from such "virtual floppy disk".
Alex
Posts: 14
Joined: Wed May 26, 2021 11:44 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by Alex »

Where can I get more information on how to do this?

Thanks!
brightcaster
Posts: 411
Joined: Fri Nov 08, 2013 10:48 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by brightcaster »

There are some threads with useful information about that:

viewtopic.php?f=3&t=697

viewtopic.php?f=3&t=520

BTW: Many 3.5" drives built during the last 15 years are HD only! That means they even lack the sensor in the HD hole and give poor results reading 720k IBM MFM floppies. With 800k APPLE GCR it's even worse!

But the Samsung SFD-321B (you mentioned,) is usually known for giving good results...

David
User avatar
mr.vince
Posts: 2198
Joined: Tue Oct 05, 2010 5:48 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by mr.vince »

The Sony MFP(?) 920 also reads Mac disks well...
SomeGuy
Posts: 321
Joined: Wed Feb 18, 2015 8:18 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by SomeGuy »

In a nutshell, you always need to test drive(s) you pick with some known good quality factory produced 800k disks (like a driver disk or application disk that has not been written to).

The results of any one specific drive may vary.

Whenever I have to archive Macintosh disks, especially damaged or heavily used disks, I usually wind up having to switch between multiple drives. I have found that Teac and Toshiba usually give the best results.

My personal recommendation is to start by making a Preservation Stream dump. Then feed it back in to the kryoflux software to decode to an image file. When errors occur, focus on just re-dumping those specific tracks. (Too often, one drive will read only the outer tracks fine, and then another drive will read only the inner tracks fine).
Alex
Posts: 14
Joined: Wed May 26, 2021 11:44 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by Alex »

Great, with cherry picking parts of "stream files, preservation" I managed to get a 100% image with 2 drives for my first floppy. The second floppy still have multiple bad reads even with 4 drives (I'm waiting for 2 new drives to arrive do see if I can squeeze more).

I have a question, what is the Drive option "Density Line Low" and "Density Line High"? Can I mix .raw files of the same floppy with either option (mixing "low" .raw and "high" .raw files in the same stream)? Because for each option I have a slight difference in read.
SomeGuy
Posts: 321
Joined: Wed Feb 18, 2015 8:18 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by SomeGuy »

That directly controls the logic level of the Density Line to the floppy drive. Usually setting Logic Low = High Density and Logic High = Low Density. Confusing, yes, but a few drives from non-IBM PC compatible system actually REVERSE their logic level signals to that line, hence why Kryoflux only refers to the logic level, rather than calling it high density/low density.

Drives use different analog signal filtering in high density and low density mode.

360k and 720k drives do not use that line at all. Most 1.44mb ignore the density line, taking input from the density notch on the floppy instead. A few 1.44mb drives use that line to set "Japanese Mode 3" which spins the disk at 360RPM instead of 300RPM.

However, I do have one 1.44mb drive (and I have been meaning to post this info somewhere, so here it is) that actually sets the density based on this line, rather than the disk notch. It is a Sony MPF520-D P/N 04H7405 FRU 93F2361. As I recall it was pulled from an IBM 486 or early Pentium 1 desktop a long time ago. It is otherwise a standard 1.44mb drive, not that oddball PS/2 stuff, and it works perfectly with the Kryoflux reading 1.44mb and 720k floppy disks.

Although this drive will not usually read "good" Mac 800k disks successfully in one pass, changing the density line will switch between either the outer or inner tracks being readable. The outer tracks are technically closer to high density. (On a real Macintosh drive, the drive rotation speed changes as it moves from track to track).

As for the .raw files, the kryoflux decoding does not actually care about that. So yes, you can mix and match those. Although with typical 1.44m drives that setting won't make any difference at all.

(An alternate way to get a normal 1.44m drive to filter a as high density is to punch a density notch in the floppy disk. Again, the kryoflux .raw files don't care and can be mix and matched.)

For log file bookkeeping purposes, and to prevent accidental "mode 3" activation, I usually do make sure to set "Line Low" for high density disks and "Line High" for low density disks, even if it gets ignored.
User avatar
IFW
Posts: 3127
Joined: Mon Nov 08, 2010 2:42 pm

Re: How to deal with bad sectors on 800K Mac floppies

Post by IFW »

SomeGuy wrote a very nice summary!
In addition, one thing that can catch you out (happened to me a few times as well) is people writing DD data to a HD disk. Discounting the fact that the magnetic properties are not ideal for such an experiment, your target system will likely be able to use such a disk (for a while) when equipped with a DD drive, since that ignores the density hole.
However once such a HD disk written on a DD drive is actually read in a HD drive, you will get random reading artefacts, pretty much what you describe above.
The solution is (if the recording is still readable at all) to simply use a small piece of dark tape over the density hole - reading errors are magically fixed.
Post Reply