Method for reading problematic disks (?)

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Joined: 18 Mar 2021, 10:28

Method for reading problematic disks (?)

Post by peo »

As described in the second post of "KryoFlux Academic", I think using the preservation format not necessarily is the best method for preserving disks that are hard to read.

Using -i0 without any guide format (as -i5 for Amiga or -i4 for standard PC disks etc) will not provide any validation of what is read from the disk, it just stores the flux stream whatever it contains.

I recently got a load of badly damaged Amiga floppies. Nothing of importance on those I created images of, but good sample disks to perform deeper preservation attempts on. I ended up writing a guide of what I learned during the attempts on these disks.

The result of the first read attempts was quite a disappointment. Of a total of 52 disks:
2 were scratching the heads so badly that I aborted the attempt after the first few tracks
7 of them were readable without any retries
14 of them were read correctly after up to 15 retries
14 read all tracks as unformatted (or not Amiga formatted)
15 had unrecoverable errors

Those 15 with the unrecoverable errors were the subjects for my experiment. The result;
8 was fully imaged as Amiga disks
6 have a few damaged tracks remaining
1 got killed in action (too many reads of the last damaged track, got that back as "unformatted" at the end)

So a summary of what I did to recover as much as possible from each of the problematic disks:
First read was with only -i5 output as I were sure they should be Amiga formatted disks
The second read was done with the same drive, but with only -i0 (output) and -i5 (for validation) and up to 50 read attempts, and I read only the problematic tracks, one by one (even if they were adjacent).
3rd read as the second but with another drive (getting more i5 valid tracks)
Further reads with other drives and switching back to those I used earlier.
Eventually all tracks were read and validated ok.

From the set of raw stream files, I created a partial ADF to combine with my first read.
After those two ADFs were combined, I had a fully valid Amiga disk image.
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Re: Method for reading problematic disks (?)

Post by IFW »

Usually what I do (if the format is completely unknown) is to check the disk surface first, clean as needed, then make a dump of the disk without a guide format, and keep the stream files - just in case they are needed later, ie disk is in very bad condition.
Find out the format from the stream dump, check the stream files using the format found (not using the disk), then redump only the tracks needed with the guide format found and clean/redump the bad tracks as needed.
This way the reading of a physical disk only happens when it is absolutely necessary.
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