Why Can't I burn 8.5GB to an 8.5GB DVD-R DL?

Question

Hi
I'm trying to archive some photos to DVD-R DL with a capacity of 8.5GB. So far I've managed to get about 7.7GB on to a single disc but can't seem to fit any more data than this onto any single disc. In fact I've tried to burn some discs (Direct from Tiger) with 6.5GB and I get a message telling me that there is "too much data" to fit on the DVD!
How can 6.5GB be 'too much' for a 8.5GB DVD?
Are the discs themselves at fault? I'm using Verbatim discs which I thought were a reputable brand.
Can anyone explain this?
Q

Answer 1

You should be able to get about 7.7 - 7.9 GB or so on the "8.5 GB" disc. It has to do with the way the disc manufacturers define a gigabyte (the hard drive manufacturers do the same).
Your 8.5 GB disc has approximately 8,500,000,000 bytes available on it. The disc manufacturers define a gigabyte as 1,000,000,000 bytes (1,000 x 1,000 x 1,000), so that's how they arrive at 8.5 GB. Computer operating systems define a gigabyte as 1,073,741,824 bytes (1,024 x 1,024 x 1,024) so when you divide 8,500,000,000 bytes by 1,073,741,824, you get 7.916 GB. Other formatting information and whatnot reduces the usable space further.
Verbatim is probably the top brand for the double/dual layer discs.
I'm not sure what you're burning onto these discs, but if you are creating DVDs to play in a standalone DVD player, you should be aware of the serious limitations that DVD-R DL discs have and DVD+R DL discs don't.