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How do 3.7GB of files = a backup > 4.2GB?


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I received a copy of Retrospect Express with my Buslink USB 2.0 hard drive, but I am actually trying to use it with a Toshiba SD-W2002 DVD-RAM drive.




Here's my question, and it's really simple ...




How is it possible that 3.7GB of files to be backed up require more space than is available on a 4.7GB DVD-RAM?




The formatted capacity of a 4.7GB DVD-RAM is 4.26GB; that is still 500MB more than I'm trying to back up, and yet along comes Retrospect, creates two 2GB volume files then a further 220MB file and promptly reports that it needs more space. Why?




And why, when I have compression set on and a mix of file types do I only ever get 0% compression reported ? And even if I accept 0% compression, I definitely can't accept a negative compression ratio that actually makes a contiguous backup volume 500MB larger than the source data!




Any help greatly appreciated, otherwise it's off to Start -> Control Panel -> Add/Remove Programs ... ;)

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Reply not needed. I see that Retrospect creates its own proprietary file system within containers on the disk, presumably to optimise subsequent updates. I still don't know about the negative compression ratio, but by turning compression off I got everything on one disk!



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