so.. can i convert the disk whit out Formating it, or move the data?
i don't have 500 GB free space on other computers on my network...
You can do it with Partition magic
Converts partitions among FAT, FAT32, and NTFS without losing data
Also have a look at the gparted live disk (FOSS) - http://gparted.sourceforge.net/livecd.php
. I have used it to convert other file systems with success, but never ntfs-to-fat so I cannot comment on if it will do the conversion without some loss, but it's worth looking at. I've used it for data recovery, partitioning and formating drives.
keep in mind that converting a 500GB partition will take quite a bit of time. It's always a good idea to backup any valuable data before modifying your drives.