26 Feb
2015
26 Feb
'15
1:21 p.m.
On 2/26/2015 1:42 PM, John Foust wrote:
While reading old Amiga floppies with the SuperCard
Pro, I have
encountered several floppies where it seems the mag-media cookie
is no longer attached to the metal hub.
I've never had luck reattaching the media to the hub when it becomes
detached. (I've used a dab of thin but sticky glue.) I chalked it up
to not being able to get the alignment perfect, but it could just be
coincidence that my media had failed at the same time.
26 Feb
26 Feb
1:29 p.m.
New subject: 3.5" floppy failure mode? Hub not attched to media.
On Thu, Feb 26, 2015 at 2:21 PM, David Schmidt <david at attglobal.net> wrote:
I've never had luck reattaching the media to the
hub when it becomes
detached. (I've used a dab of thin but sticky glue.) I chalked it up to
not being able to get the alignment perfect,
That seems quite likely. The track pitch is less than 0.2 mm. A
positioning error of less than 0.1 mm when reattaching the hub will
render the disk unreadable.
2:14 p.m.
New subject: 3.5" floppy failure mode? Hub not attched to media.
> I've never had luck reattaching the media to
the hub when it becomes
> detached. (I've used a dab of thin but sticky glue.) I chalked it up to
> not being able to get the alignment perfect,
On Thu, 26 Feb 2015, Eric Smith wrote:
That seems quite likely. The track pitch is less than
0.2 mm. A
positioning error of less than 0.1 mm when reattaching the hub will
render the disk unreadable.
Same basic problem occurred occasionally in >3.5" media with misclamping.
That could be confirmed by reformatting a track and then checking it.
(Outer track preference in allocation would make innermost tracks the
least risky to sacrifice)
But confirmation of the specific problem doesn't solve it.
Idle speculation:
If it were truly important to fix it, you would need to build a hub
assembly that could be offset in two dimensions, such as a 4 jaw lathe
chuck. Then use disclosing fluid and a good magnifier (reticle?) mounted
to the drive frame or head carriage, and turn the disk and look at the
amount of runout, readjust, recheck, readjust, recheck. The idea would be
to offset the center of the hub relative to the center of the spindle.
NO, I have not ever tried to do it.
If anybody ever succeeds at optical reading of a non-rotating disk, ...
--
Grumpy Ol' Fred cisin at xenosoft.com
2:26 p.m.
New subject: 3.5" floppy failure mode? Hub not attched to media.
On 02/26/2015 01:29 PM, Eric Smith wrote:
On Thu, Feb 26, 2015 at 2:21 PM, David Schmidt
<david at attglobal.net> wrote:
It isn't particularly the track pitch that gets to be critical, but the
radial positioning of the hub that defines the index position. If you
can read disks without relying on the index pulse, you may have better luck.
--Chuck
I've never had luck reattaching the media to
the hub when it becomes
detached. (I've used a dab of thin but sticky glue.) I chalked it up to
not being able to get the alignment perfect,
That seems quite likely. The track pitch is less than 0.2 mm. A
positioning error of less than 0.1 mm when reattaching the hub will
render the disk unreadable. 
27 Feb
27 Feb
5:54 a.m.
New subject: 3.5" floppy failure mode? Hub not attched to media.
On Thu, Feb 26, 2015 at 02:26:05PM -0800, Chuck Guzis wrote:
[...]
It isn't particularly the track pitch that gets to
be critical, but the
radial positioning of the hub that defines the index position. If you can
read disks without relying on the index pulse, you may have better luck.
The original poster is reading Amiga disks. Although the Amiga hardware can
sense index pulses, the native format ("trackdisk.device") does not make use of
them. When the Amiga writes a sector, it actually does a read-modify-format of
the whole track. This is why the sectors are all bunched together with a single
large inter-sector gap per track, and also why "You MUST replace Volume foobar
in DF0:" due to ejecting a disk mid-write, as the whole track is now corrupted
and needs to be re-written.
I've not disassembled mfm.device to see if it does the same thing with MS-DOS
disks, but I'd be surprised if it didn't.
As to the disintegrating disk, there's probably some cute bit-nibbling trick
that can be performed by bit-banging the Amiga's disk countroller to retrieve
the data even if the media is misaligned due to re-gluing, but it's not worth
the effort for a disk that's clearly badly-made and was probably unreadable
even before it fell apart.
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abuse＠cabal.org.uk -
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