Removable digital storage media… ah, yes, it’s something we no longer use nowadays in modern desktop computing. Only in the halls of big datacenters might you find removable storage media nowadays. But, if you’re really into 1980s and 1990s era vintage computing, removable media is a major element of this genre. Additionally, errors on removable media were also a somewhat rare but occasional occurrence. So, when you take out your physical media that is in error and look at it, how do you know how to deal with the error? First, you need to know the geometry that the data is logically stored on the physical media.
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Floppy disks and hard disks: Both floppy disks and hard disks are formatted via the cylinder, head, sector (CHS) methodology. Cylinders/tracks are concentric rings. The logical first cylinder is always the outermost ring. Each head is one side of a circular disk. Side A of a floppy disk is the “top” side, as seen when you insert the disk into the drive, and side B is the “bottom” side.
The cylinders/tracks on most floppy disks are divided up into the same number of sectors regardless of where the cylinder appears on hte disk surface. For example, IBM-formatted disks have the same number of sectors per track on all tracks on a disk, but Apple-formatted disks, format more sectors on outermost tracks.
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CD-ROMs and other optical discs: The data is written on a spiral track that starts on the inner edge of the disc, spirals counterclockwise, and ends on the outer-edge of the disc.
- Phonograph records: I include this one here simply because it often times heleps cause confusion to people when they think about floppy disks, CD-ROMs, and phonograph records together. I mean, come on, they’re all discs, shouldn’t they all be treated relatively the same? Nope. The spiral track on a phonograph records begins at the outer-edge of the disc, spirals counterclockwise, and ends on the inner edge of the disc.
20190508/https://en.wikipedia.org/wiki/Compact_disc
20190508/https://en.wikipedia.org/wiki/Floppy_disk
20190508/https://en.wikipedia.org/wiki/Phonograph_record
20190508/DuckDuckGo floppy disk first sector outer edge inner edge
20190508/https://superuser.com/questions/643013/are-partitions-to-the-inner-outer-edge-significantly-faster/643634
So, now you know where to look for errors in case you find them. Have you been flicking your wrist when handling a 3.5 inch floppy disk, and now your computer claims it can’t read the boot sector? Ah, that shaking around inside the plastic cartridge must have done it to the outer-edge of the disk. No worries, the rest of the inner tracks of the disk should be readable.
Do you have an CD-ROM where you are getting read errors near the beginning? Check the inner-most ring of your optical disc for scratches, discoloration (for CD-R), or other deformities. Then, you can figure out how to treat the disc from there.
Do you have a read-error near the end of your CD-ROM’s logical data? First of all, check what the total size of your ISO filesystem is. If you use less than 700 MB of data, then the logical “end” will be proportionally closer to the interior of the disc. Once you identify the logical end of the disc, you can look around there for deformities that might cause read errors and figure out how to treat the disc from there.