Strange disk format free space amounts?

[bfollowell]

I have some old disks I'm trying to format so that I can checkout the disk drives on an old 520STM and 1040STF that I plan to sell. These are old HD disks from my PC days. I've just taped over the hole and am formatting them as 720KB disks. I know that isn't the best for longevity, but this is just for temporary testing, not long-term storage, so I'm not overly worried. I have very few remaining disks, and even fewer of those are 720KB disks that I can sacrifice. I've had quite a few fail, but several have surprisingly formatted. The strange thing is that they all seem to be formatting to differing amounts. One was 720,896 bytes. Another one was 723,968. Still another was 726,016. Of the three that I've successfully formatted so far, all three are completely different. I understand the ones that just fail, but why would a disk successfully formatted as a double-sided disk not read the same amount of space as another disk formatted the same way?

 

Thanks.

 

[ParanoidLittleMan]

Because format performs test for every sector on disk - and if it is bad, it will be marked as such (practically as used one, not free for data) - and free capacity will be less.

Depending on count of problematic sectors detected.

Btw. same happens with modern Flash cards after manufacturing memory chip(s) - they go under test, and bad sectors will be marked, and capacity lower.

Look just real capacity of some Flash card - writes 8 GB for instance, while free space is in range 7.5 GB - better by good brands. And will hardly find 2 cards with exact same capacity. 

Longevity ? Would not use that word when talking about floppies now. Today needed to write again some short files on floppy, before it to format it, after only about 1 month. And it was DD, not HD floppy. Did 2 - for case - really not fun to dig out old PC with floppy drive because couple files ...

Surely, manufactured some 25-30 years ago - so it may be considered as long lifetime, only that now is in grave with 1 leg ?