As FurryNutz says, on a FAT32 formatted partition you are limited to files less than 4GB which means there is no way you can copy full DVDs to such a drive.
In your case where you try to copy a file larger than 4GB it will copy the first 4GB then try to copy the remainder but the file system will not accept it therefore the crash. It is even possible to crash the NAS in that condition depending on how much larger than 4GB the file is. At 4.3GB you are on the borderline and I hope you have a tested backup of your NAS should there be a kernel panic type crash that damages the file structure.
Regarding your slow NTFS USB hard drive, is it a USB 1.1 or 2? USB1.1 will be very slow on the NAS (as will a USB3 for some reason, at least here they are).