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garyasta
15-07-2004, 03:16 PM
How does data get moved around on a CD or floppy?

I can understand how data gets shifted and dropped in spare space on a HD and then requires defragmenting to put back in order but what happens when data on a floppy or CD is changed or saved in a different format?

Cheers
Gary

Billy T
15-07-2004, 06:13 PM
Hi Gary

A floppy behaves the same as any other magnetic disk, except with lower capacity and less room to shuffle data.

CD-R or CD-RW just writes to another unused location when data is saved, updated or the file format is changed.

A CD-RW could possibly be defragged as it is sort of supposed to act like a high capacity floppy disk. It shouldn't get fragmented anyway though, as it is supposed to be burned in contiguous form.

If you were to burn a bit by bit dump from a fragmented disk, I suppose that theoretically it would be written as one large fragmented file. It would probably lack any FAT to recover the file fragments though.

It is an interesting thought though, that bears thinking about from a purely academic viewpoint (at which time I bow out for lack of both academia and viewpoint) so hopefully Godfather or other oracle will pop up and elucidate.

Cheers

Billy 8-{)

godfather
15-07-2004, 06:40 PM
"A CD-RW could possibly be defragged as it is sort of supposed to act like a high capacity floppy disk. It shouldn't get fragmented anyway though, as it is supposed to be burned in contiguous form."

Not in their intended form.

As I understand it:

A CD-RW is not a CD Read/Write device, its a CD Re-Write device.
Subtle difference but it means that they behave exactly as a normal CD-R where data is "write once and read many times"(WORM).

The Re-Write really only means the CD can be totally erased and used again.

The implementation of UDF packet writers gets around this to an extent, but it appears to introduce a high level of unreliability in data storage, relative to a HDD. Packet writers (InCD and Direct CD etc) appear to be able to selectively write and erase sections of a CD. Like a slow HDD. Just don't do it with valuable or irreplaceable data in my experience!

Multisession CD-R appear to re-write (to a new area of the CD each time) the Table of Contents whan more data is added. This does introduce the danger that any corruption at this point can render all the data unreadable, not just the last written. Unsure if they destroy the old TOC.

I will only use CD-R, and not multisession for secure storage.

Apart from that, data should be contiguous on a CD-R, I imagine, as there probably cannot be "gaps" available that other data could drop into. Even if there was, the TOC could show where the data was, and what other segments were used just like a HDD? Once written it cannot shift anyway, so defragging would be pointless.

Graham L
16-07-2004, 01:54 PM
The old TOC remains there, it's just "invisible", like a "deleted" file in a DOS disk. Some software can find the old ones, so previous versions of a file can be retrieved. So you can't necessarily destroy the evidence. :D

garyasta
20-07-2004, 05:29 PM
Hi Guys

Thanks for your responses.

Cheers
Gary