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whitelightning
Asked: 2019-02-22 15:13:41 +0800 CST

Blank 700 MB CD shows as 438.1 MB free

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I inserted a 700MB blank CD into the optical drive. I then right clicked on an ISO file that is 298MB and selected write to disk. The window that came up showed 438.1 MB free. Why isn't it 700MB? I inserted it into a Windows machine as an "experiment" and Windows showed 702MB.

burning 18.04

2 Answers

  1. Let's do the math.

    700MB - 298MB ~= 402MB, so 438MB is about what I'd expect.

    Why does it vary?

    Different systems report different MBs. Some will use decimal megabytes 1MB = 1,000,000 bytes. Some will use binary megabytes 1MB = 2^20 = 1,048,576 bytes. So your CD size may be underreported. It's not uncommon to buy a 700MB CD (binary) and find it's 740MB (decimal).

    Also, because CD and DVD data is stored in blocks of 2048 bytes, not all of those blocks will be full. For comparison, your platter disk or SSD will, most likely, be storing in blocks of 512 bytes.

    Lastly, CDs are designed using a special format (called Red Book) which is designed for specifically for digital audio and can, under certain circumstances, be denser than a regular disk.

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  2. Best Answer

    That is actually how the app Brasero works, if that is what you are using to burn the ISO or if that is what is installed by default.

    Here is Brasero's image burning dialog before an ISO file is selected:

    enter image description here

    as we can see the full available amount of the disc is shown.

    After we select an ISO file:

    enter image description here

    It is now showing that only 36.6 MB is available, as this is the available amount after writing the ISO to the disc. Unfortunately, Brasero will close this disc and the amount can no longer be used after the ISO is written to the disc.

    Hope this helps shed a little more light.

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