iTunes has a great feature: iTunes Backup. You can select a playlist or the whole library and can back it up to multiple DVDs. The problem is with the restoration. I’ve had this happen twice. In one case, I had the discs and a Mac and one disc would keep failing. I dumped the contents to a hard drive and copied them into iTunes that way. The other situation was with a Windows machine. I’m wondering if in this situation it was a filename-length issue. The files were created on Mac-formatted drive and he was restoring to FAT32. I overcame that with the hard-drive method. iTunes won’t tell you which file failed, either, so a whole disc is suspect when it fails.
The files themselves aren’t faulty. When you insert an iTunes Backup disc, it will appear in iTunes and you’ll be asked if you want to restore. If you ignore this, you’ll see that it’s a standard disc in the Finder or Windows Explorer. iTunes adds a little magic somehow to identify it as an iTunes Backup disc. To do the hard-drive trick mentioned above, I simply copied to a hard drive, then copied from the hard drive to iTunes. This will definitely bypass the disk-format issue if you use the same drive for both the copy and iTunes. If a file fails, your OS should give you some sort of explicit error for a specific file that you can correct, like “filename too long”.
The other annoying thing about this feature is that it mixes up all the content, so you may have the tracks of a single album strewn over more than one disc. My only theory as to why is that iTunes is trying to maximise how much it can get on a single disc and sticking to a strictly sequential order may leave unused space. This doesn’t matter if you intend to restore the lot and the feature works!
So Apple has a very good feature for the initial part of the operation, but the critical part, restoration, is sadly lacking. Use with care.