I'm not an expert on the conversion process, but it may be that your existing volume has some files or bad sectors located in areas that would be reserved under NTFS and this is preventing the conversion.
Do you have another drive of about 200GB or so? If you do, you can copy all your files to that drive, reformat your entire 1TB drive from scratch as NTFS, and then copy the files back again.
Edit - yes, I see that's what the pjmelect's link suggests. Try chkdsk first, and if that doesn't work you'll have to do the copy to another disk.