You probably shouldn't be mounting these devices straight to /mnt. It should have an extension like /mnt/sda1 or /mnt/ntfs or something. Do you seriously have a hard disk formatted for DOS? AFAIK, the only thing close to DOS formatting we have today is vfat, or FAT32. Is that what it's formatted as? if so, change your filesystem type from msdos to vfat.
Captive is not worth working with. It's buggy and it'll probably crash your filesystem. If you have to have a hard disk that can be used by Linux AND Windows XP, use FAT32. It works fine.