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Isn't it the case that a DVD can only hold at most 4.7G of data? How can Apple fit these data into a DVD?

I discovered it when I tried to restore the install disk image to a USB flash drive.

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up vote 5 down vote accepted

Quoting Wikipedia...

Dual-layer recording (sometimes also known as double-layer recording) allows DVD-R and DVD+R discs to store significantly more data—up to 8.54 gigabytes per disc, compared with 4.7 gigabytes for single-layer discs.

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+1 - Dual-Layer vs. Single-Layer is it. –  afrazier Jun 14 '11 at 14:08
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These are special types of DVD's. You see it sometimes when you try to rip a film from a DVD. There are DVD's wich hold more than 4,7GB of data. Because 4,7GB is not enough, or otherwise to prevent the DVD from being copied. I've also got an old PC-game of Rayman here, wich is just a CD (it works in a CD-ROM-drive wich can't read DVD's, so no, I'm not mistaken), except that it holds 2,2GB of data.

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