Ask Slashdot: Recovering Data From 20-Year-Old Diskettes? 375
First time accepted submitter Zilog writes "The end of the 3.5-inch floppy and the disappearance of associated drives showed to me the need to backup the tens of diskettes that accompanied my youth. Carefully preserved, these diskettes have proved readable for the most part — while some are approaching 20 years old. However, some diskettes have shown surface defects in areas with compressed archives (zip). Any ideas on how to recover (as much as possible) these bad sectors?"
Re:The wisdom of using compression in archives (Score:5, Insightful)
Re:The wisdom of using compression in archives (Score:5, Insightful)
don't
Clever, but ...
1) an error inside a zip file (or any compressed archive format) means that any file inside the archive that is stored on the corrupted part of the disk is corrupted. Compare this to the situation without a zip file -- any file stored on the corrupted part of the disk is corrupted.
The rest of of the files in the zip file, the ones stored on parts of the disk that aren't corrupted, are recoverable.
Now, if the table of contents of the zip file is corrupted but the data itself is OK, then you can still recover the data, but it becomes more difficult -- and you'll lose the names of the files. Compare this to the situation where the directory data for the diskette is corrupted but the rest of the disk is fine -- same thing.
The only important difference between files stored in a zip file that are corrupted and files just on the disk that are corrupted here is that if there's an error in the middle of the compressed data in the zip file, that means the file is corrupt from that point on for a file compressed in a zip archive, but that only those blocks are corrupt in the case of a file just on the disk. Does it make a difference how much of the file is corrupt? Maybe. If it's a text file that can't be recovered, yes. But if it's an executable or some data file that just can't be loaded either way -- not really.
2) the zip archive means that the data probably requires less space on the disk. It may not have even fit on the floppy at all without compression. That alone is a pretty important reason to use compression in archives. If you can cram twice as much data on a single floppy -- you could possibly store it on two floppies instead, giving you a backup in case one floppy fails.
3) being compressed means that the file took less space on the disk -- therefore the odds of one of it's blocks becoming corrupt goes down similarly. (Assuming that just a handful of blocks have become corrupt. If the whole disk goes bad, you're screwed either way. Of course, with compression, losing an entire disk means you've lost even more data. But I'm not sure using 360 KB floppies rather than 1.4 MB floppies is really an appropriate data saving measure either.)
4) compressed archives almost always have checksums of some sort which will tell you if their data is corrupted. Of course, some archive formats that don't involve compression have checksums too -- but many don't.
It's very good to be able to tell quickly and programatically if your data has been corrupted.
Re:Brute Force? (Score:4, Insightful)
A floppy disk sector is 512 bytes, so even with just a single unreadable sector there are 256^512 possible combinations, more than there are atoms in the universe.
Re:Clean and align (Score:1, Insightful)
Great post, but I want to point out to everyone reading it that he said "UNdenatured ethanol" and that means (if you keep reading) Everclear which is sold in the liquor section. If you drink denatured ethanol, you will consume enough methanol to go blind and possibly die.
I agree with the shot idea, but didn't want any idiots out there skimming the post and fucking themselves up.
Fags.