turalyon is correct. FAT32 file system only supports a maximum of 4GB files size. I am not sure about the DVD Decrypter message but I believe it's just a warning that if you dump a disc to an ISO image format larger than 4GB and write it to the mentioned volumes, the DVD Decrypter will split the image file.
Look here:
FAT32
I would suggest formatting the FAT32 file systems to NTFS or ext3. NTFS can support up to 16GB max filesize and ext3 can support max of 2TB filesize.
I would, however, only format the volume that needs to store big file sizes. This would lessen the damage on the disk if you are gonna pull out all the files, format, then put back the files. Not a healthy activity for hdd.