Like has been said - the most logical answer is that your computer's hard drive (the one you're trying to write to anyway) is most likely formatted in FAT32 and not NTFS. Are you running Windows XP or an older version of Windows? To find out if the drive is formatted using FAT32; go to "My Computer", right-click on the drive you want to rip to and click Properties. There should be something that says "File System:", whatever it says there is what the drive's formatted with.

Now, the reason you can't use FAT32 is that it can't handle really large files. I thought it could handle files up to 2GB and if you're ripping in IFO mode it shouldn't be a problem. If you're ripping in ISO mode... well then it won't work because most DVDs are more than 2GB.