I bought this spindle of 25 Memorex DVD+R DL at Ebay, which were suppose to be 8.5 GB. I try to burn a 7.8 GB image and the software said not enough space. I went to properties at my computer and it said 6.5 GB total capacity. Am I doing something wrong? May it means that they are bad copies of the original? Is this normal for Memorex? If it is, is there a more thoughtful brand?
The two layers on the DL discs use pits that are 10% larger than those used on standard DVDs, so instead of 2 x 4.7 GB = 9.4 GB, a DL disc holds only 8.55 GB or so. But that is in decimal terminology where "kilo" ("thousand" in Greek) actually means a thousand. Computers can only count in binary mode, and two to the tenth is 1,024, not one thousand. Early computer technicians foolishly applied Greek words to numbers greater than what the actual meanings were. In the mid-1950s, however, IBM standardized on using decimal terms for storage media and binary terms for computer calculations; but few people noticed. Today, every multiple of a thousand for data storage equal 1,000; but every multiple of a thousand for the computer's view is only 976.6 (1000/1024). A computer sees the 8.55 billion bytes of DL capacity as "7.96 GB," but there actually is room for 8.55 billion bytes.
It gets more complicated when you are storing data because the computer allocates a good deal of available capacity for redundant information used for error correction. (Video and audio recording use less overhead for error correction than data recording.) If your computer is reserving 6.5 GB of capacity for data storage, for data recording software is allocating about 1.46 GB for redundant information and/or formatting. That seems excessive; but you don't mention what recording software you have, so it's difficult to say. (Some recording software intentionally limits capacities to avoid recording to the outer edge of discs, the area where discs are most prone to problems.) The limitations you are seeing are not due to the discs' reporting but to the differences in binary/decimal calculations and, probably, to protections added by the recording software.
They are both right. There is not enough space on a DL disc to record a 7.8-GB image. You will have to compress the image somehow or alter it so that it is not quite so large. Another alternative is to span it to 2 DVDs rather than record it to a DL disc. That may be the best choice since DL discs inherently have bursts of errors at the layer change when the pickup head refocuses and reverses direction.
