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I've never come across an unreadable DVD disc once it's been burned, but there is a situation where multi-session data cannot be recorded if the previous session(s) has taken up most of the space. I forget the precise figure - something around 3.6 or 4gb. I think this limitation is probably software related.
I've also come across a problem with early 1x recordable discs. If such a disc is not fully compatible with your burner, or due to quality variation a disc is not quite up to spec and cannot record as fast as 1x, then you'll likely produce a coaster. My software, Record Now DX, appears to do a quick scan of the disc before recording begins. Sometimes a 4x disc will drop down to 2x (I assume because of quality variation - I use cheap discs ), but a 1x has no lower speed to drop to.
I started recording DVD-Rs back when the cheapest burner (Pioneer 103) was $450 and 2x max. Around 3 month after buying the recorder, I burned 72 DVDs on generic media. Of those 72 discs, I had problems getting data back off of around 5. About a year later, I burned 50 discs with the recovered data on a different generic media. Of those 50, 2 of them were totally unreadable, another 2 are readable except for the last file on the disc, and around 10 slow down to around 1/100 speed after about 4GB. (the files were DV AVI files ranging from 100K to over 4GB in size recorded as single session UDF format discs). Note that ALL these discs were readable right after they were written. The problems came later. (Around 2 of each batch had problems caused by minor scratches. of those 4, 2 were recoverable with cleaning and a long session of recovering data with CD/DVD Diagnostic (long as in 12 hours to read 500MB of data). (CD/DVD Diagnostic is also sold as DVD X Rescue.)
Since those problems, I have switched to Ritek G04, and after over 100 discs, have had no problems with reliability since. (and since Ritek G04 discs are only around 10 cents more expensive than generic discs, I won't risk my valuable data to anything lesser)
Another thing to note is that DVD burners have become better over time. The current burners do a very good job of burning a good disc even with poor media.
You Also need to qualify DVD+R from DVD-R as the performance and reliability of discs depends on the format as well as the maker. DVD-R has both the best and worst discs available, while DVD+R discs are all good, but not great (you probably won't have the recording problems with generic DVD+R discs that you have with generic DVD-R discs, but you also won't get the readability and longevity you would with top of the line DVD-R discs.)
P.S. On a weird note - normally if you see a discolored spot on the bottom of a burned DVD, you had dust on the DVD, and you probably burned a coaster. I have one DVD with a severe discoloration in a large irregular spot, but the DVD was completely readable and showed no slowdown at all.[/font]