That's strange. Here are some things you can do to try to find where and what is the error.
1st: What cat outputs when trying to read the file with the device name?
2nd: Did you tried mounting anywhere else? Try creating a specific folder to this device (lets say, /media/ntfs-partition) and mounting it there
3rd: Did you tried mounting it with the native mount command (it mounts the device on read-only mode, but if the partition has data inside it, you should be able to read it.) There's probably an error with ntfs-3g or a rw mount(?) (use # mount -t ntfs /dev/device mounting-point
)
4th: Can you mount it correctly inside another operating system? Linux, Windows, maybe?
5th: If everything fails, try backing up the data of the partition/device (if the above test succeeds) and formatting it on FreeBSD (you can use gpart or fdisk to do that) and try mounting it again
6th: The last one, just to make sure its not a simple mistake: Do you use the device name or the partition name to the command? I have no doubt of your knowledge, but the problem can be easier to solve if all the right questions are made. To mount a device you should specify the device name (lets say /dev/ada0) and the partition (s for slices and p for partitions, so /dev/ada0s1 first slice and /dev/ada0p1 first partition. I'm not sure but I think a disk cannot have both).
Good luck, I hope you find the right answer/solution. If something interesting or at least strange appears on the above (or on your own) tests, post it here. I'll be glad to help you trying to solve your problem. Try asking a mailing list too, you can find your solution there.