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Is possible rebuild a cut floppy disk & retrieve data?

Computer forensics discussion. Please ensure that your post is not better suited to one of the forums below (if it is, please post it there instead!)
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Wed Jan 16, 2013 12:31 pm

- PaulSanderson
- mscotgrove
on double density disks, there is not always a clock bit.


You sure about this? Single or double density the encoding is still MFM (assumimg a PC and not GCR on a Mac) and MFM calls for clock bits always. It's been a long time since I read a floppy at the raw level but I dont recall any difference in the way I read and pieced together double/single density disks.


It has been a long time too for me working on floppies - the following is from memory.

Single density ALWAYs has clock bit

MFM has a clock bit if there is no precceding data bit or following data bit - I think

M2FM was slightly different logic, but along the same lines where clock bits were added if there were no data bits. I think Intel were the only people who ever used M2FM

GCR was often 5 bits of disk data for 4 bits of actual data - make sure there were never more than 2 zeros in a row.

400K/800K Macs were GCR, but with 5 different recording rates for each group of 16 tracks.

Sirius and Commodore were all GCR, and some Commodore were 100tpi
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Michael Cotgrove
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mscotgrove
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Wed Jan 16, 2013 1:49 pm

A very quick bit of googling - I think you may be on about MMFM dropping the odd bit. I am pretty certain that both single density and double density floppies used standard MFM - it was just packed in twice as densely (stating the obvious).
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Paul Sanderson
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PaulSanderson
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Wed Jan 16, 2013 6:03 pm

I disagree

Only SD always has a clock bit
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Michael Cotgrove
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cnwrecovery.blogspot.com/ 

mscotgrove
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Thu Jan 17, 2013 6:39 am

en.wikipedia.org/wiki/...Modulation

Single density is FM. Typically this died with 8" IBM disks

Some very early 5.25 disk were single density, but most were 40 track double density (360K) or 80 track, High density, 1.2MB

I am not aware of a 3.5" disk that was ever single density, always double density (720K)or high density (1.4M) Extra density was 2.8M but never caught on.


Double and high density all use MFM (with conditional clock bits).
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Michael Cotgrove
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cnwrecovery.blogspot.com/ 

mscotgrove
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Thu Jan 17, 2013 7:22 am

Sorry I was just talking about 3.5" disks - I haven't seen a 5.25 for about 15 years
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Paul Sanderson
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Re: Is possible rebuild a cut floppy disk & retrieve data?

Post Posted: Sun Jan 20, 2013 5:16 pm

- keydet89
It's been possible since the '90s:
www.csoonline.com/arti...f-evidence


Nice! The snodgrass case is THE case that made me interested in forensics in the first place. The solution was brilliant, I remember that they developed a method to warm up the cut up and bent pieces with a soldering iron without melting it by attaching a metal to it that would reduce the heat just enough to make the disc go back to normal - absolutely brilliant!  

MDCR
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