RW sessions on a dvd is very dependent upon the quality of the medium used which is why multi-session is to be avoided.
IIRC one laser is used for read and one for write, this is altering the physical characteristics of the surface layer of the medium. So dependent on medium quality, laser* quality you have a very unpredicatable response - it should all work but experience going over the years is not something I would rely on or practice especially with the cost of the medium being so cheap.
You can always mutli-write - I am not saying dont, but your putting your hands in economical technology. If you pay more for the medium and the hardware you will more than likely get more predicatable results. Which is why most people stick to write once read many mediums for external disks.
Have you tried copying all bar the one folder to one dvd and the rest to the next ? - you have to be careful when writing files to a disk as it can be misleading on storage capacity. For example:.....
If you have 1024 files that are 1 byte in size you would think it would take 1024 bytes to store on a disk. However if you are using a FAT32 disk the minmum unit of a file size is 32k so a 1 byte file will occupy 32k so the total storage spaced required will be 1024x32k.
The point of the above is that if your storing hundreds of files, whilst it may appear that the space consumed is less than 4.3gb that the total required storage space given the sector sizes of the dvd is more than that. Rather than go into the details of storage and how it is done - either compress into one big file or multiple big files with winzip or equivalent or spread it over many disks.
The other thing to try of course if it is not the storage being the problem is copy what you have validated to one disk - close of the session and you have that secure - start the next disk and if you repeatidly fail on the last folder it will be a corruption there somewhere so either write off the whole directory or identify which file is lost by copying a file at a time till it fails, it can be long and laborious but it depends how much the contents are worth to you.