You Cannot Put 700MB of Data on a 700MB RW Disc: Why?

You Cannot Put 700MB of Data on a 700MB RW Disc: Why?
(This also pertains to DVDs and Solid State Flash Media)

[The point of this Tip is to help you understand in the broadest of terms why you cannot use up all the space on your CD or DVD RW media and your CD-Rs and DVD-Rs as well as Flash Media.]

Lets start with RW media where this causes the most confusion:
You will never be able to format the entire 700MB of any RW media. The same is true for DVD RW media. This is because the formatting requires a file allocation table (FAT) be created, the formatted disc will be segmented into cylinders (circular sections of the disc) referred to as tracks and that is further divided into Sectors. The FAT contains columns and rows that containing information on exactly where data is stored by referencing track and sector within the FAT. The file may span many sectors and be across several tracks. The data and the table takes up a lot of room. A good and easy example of this is a floppy disk. If you have a floppy disk you will notice on the diskette drawer slide it states it is a 2MB diskette, however, when formatted you only have roughly 1.44MB of usable addressable space. The same is true for any formatted RW CD or DVD.

Can this be Different between Media Maker Companies?
Simply stated: No.
Just how much space is available after formatting a RW disc on your system sometimes depends in part on the software used to format the disc. But this will never be different enough between media makers to amount to much. The exception is preformatted discs you may purchase off the shelf. Those preformatted disc may vary from maker to maker.

How about Software driven Burner Programs?
In CD RWs, for example only, the common assumption is that 545MB will be available for data storage. Every software program may be slightly different; each software burner producer reserves the same, more or less space than another. This difference will be slight, if noticeable at all.

How about CD/DVD-Rs then?
Additionally, but aside from RW media. You cannot place 700MB of data on a 700MB CD-R. Room is included on the media for file sector information and label data for Start of File and End of File markers. Each piece of data that comprises a file on the disc, RW or R, has information that is part of a label such as (although greatly simplified) part 2 of 200. I'm not overly simplifying this by example but it is close enough for an average layman to get a grasp of how this works. Each part of a file broken down by sector size (more on this in the next paragraph) on your disc has a marker to indicate which part of the whole it comprises. Your computer uses this data to reconstruct the file when you open it or copy and paste the file. When moving or sending files these parts of a file are referred to as packets.

Space Loss From Sector Size
Another reason for 'space lossage' on RW media is that when formatted, each sector may be 8K, 16K or 32K in size. However, the last sector in a string of sectors for a file rarely fills in the full 8 / 16K or 32K of space. Lets say, for example, that the last sector only has 4K of data and this is an actual average for your RW disc. You end up loosing 4,12 or 28K of space for each and every file on the disc. So in reality you have the 545MB of available space plus the loss of data in the last sector of a series of file segments. When there are a lot of file written to a RW disc, the lossage adds up real fast. Does this also apply to CD-R media? Yes, and can have a dramatic affect. On a 700MB CD-R you will never be able to put 700MB of data on the disc for this reason.

The Same is Also True for Hard Drives on your System
Another good example of this loss of storage size as advertised is with a Computer Hard Drive. A 500GB Hard Drive will not even come anywhere close to the advertised usable space once it is formatted, but when you see your drive in the System, it will be labeled and described as a 500GB drive.

How about Music CD and DVD MP3s?
Music CDs are quite a bit different but and not the same for DVD MP3 discs which are data discs.
When you purchase a CD Disc labeled as a Music CD, it is preformatted for music only. And I'll describe this method shortly. And as a result will not store data.

Music tracks on a Music CD are laid down much like a vinyl album, in one continuous groove that swirls across the entire disc media. At the beginning and end of each track on a Music CD is data, called Meta Data that contains information about the track title and artist/album. As just described for each track holding data pertaining to track title and artist/album, you cannot fit exactly 700MB of music you listen to later. Additionally, the industry standard is to included two seconds of silence between tracks titles, but still on the same continuous groove that comprises the entire Music CD.

Is this True for Solid State Removable Disks like Compact Flash?
Yes, and for the same reason already stated above pertaining to CD/DVD-RWs. When the Flash Media is formatted it also contains a FAT and the media is formatted with Tracks and Sectors. Files written have to have package labels and are formatted into there sectors and where the last sector is occupied with data that is usually less than the sector size and thus it will never be filled up to its maximum capacity.



[As stated earlier, the point of this Tip is to help you understand in the broadest of terms why you cannot use up all the space on your CD or DVD RW media and your CD-Rs and DVD-Rs as well as Flash Media.]


Authors note: As needed, for corrections or additions to this topic I will add notes to this Tip below as comments.
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