This isn't a request for info, it's just me explaining my experiences in the hope it helps other people. (And maybe me when I forget all this)

I have a few old computers that use DSDD disks. In the PC world they're known as 720K disks. They're the ones without the hold punched, with the hole punched they would be HD.

I have bought various job lots on ebay in the UK to see if I can get them working. Watching an Adrian Black video pointed me to a bit of software that looked handy. https://www.youtube.com/watch?v=iGLE0J0sZOQ&t=2139s and https://www.youtube.com/redirect?event=video_ … stival_FORMQ172

I have a P3 machine with 2 3.5" drives in it and I used it in DOS mode with this bit of software, it works great.

I also asked a friend who runs a Youtube channel "Mark Fixes Stuff" for his general information on disks as he worked in a disk duplication centre for a number of years, and he's generally excellent at everything.

He explained that disks used as cover disks (disks that came free with magazines) were typically of the lowest quality. He said that they regularly cleaned the heads on the duplication machines, and eventually the heads would wear down.

He also confirmed that it is unwise to use HD disks as DD disks by covering the hole. The disks themselves have a different grain of coating. It's quite a complicated explanation and not one I feel I could properly explain, but I trust his opinion on it.

I already had an expectation that disks would degrade to a varying degree based on how well they were stored. That changes in heat and moisture would have an effect on them.

So here's what I found out

* A fully functional disk for my purposes has no bad sectors, most of what I'll do with them will be writing images to them so a bad sector means I can't use them. If you're using disks on a PC then you can use bad sectored disks for file transfers but not images
* Some disks were so faulty that they did not make any progress on the format at all before being marked as having an error
* Other disks were faulty at some point on the pass, some might make it 90% of the way there before failing
* Some disks that passed had one or more points in the format where a retry was required
* Almost all disks that failed had some retried areas that did then pass before coming to another point at which they failed
* Cover disks were almost all faulty
* Disks that looked physically dirty typically failed
* Branded disks worked better on average than unbranded
* But by branded I mean Verbatim etc. Branded "own label" type disks did not fare any better than unbranded
* Some entire batches were useless. I had 1 disk out of 30 working before I discarded and entire batch
* One batch had almost every disk working
* Another batch had 30% working
* Another batch had 50% working
* Some batches of disks were all non-functional
* I tested 420 disks and I now have around 150 working disks
* I gave each disk a quick inspection behind the metal shutter for any marks or mould, if anything was present I did not try the disk
* It is possible to repair/restore a disk using IPA and a cotton bud (Q tip in the US). I don't know if this is permanent. But for disks I want to use as blank disks I felt it was unwise
* Some disks killed the drives until I cleaned them with IPA

As for methodology, if a disk failed I would try it in the other drive, I frequently used IPA to clean the drive heads, especially if they failed on disks. I would check that the drives were working properly with known working disks. And especially often if this was in a batch of disks that did not work.

I have 30 NOS sealed DSDD disks and I'm not sure if I should be testing them to see if they work, or leaving them sealed until I am ready to use them. Until I decide I'll leave them sealed.

I also don't know if a tested, formatted old reused disk is really any different to a NOS sealed disk that's been tested. And I don't have any way to tell at present.

I have some HD disks to try, but I've been able to get a few hundred NOS sealed ones, so I'm less concerned about those.

I also have 150 3" disks I need to deal with at some point, but I can't use a PC and DOS to test those, or at least not with that software.

I hope this helps people.