Hi!
Using GParted, you can delete your NTFS partitons on this external hard drive and create a new FAT32 partition. However, you can NOT convert the NTFS file system into FAT32 (so you won't lose the data stored on the drive)!
So you best copy all data from the external drive to another disk, then boot up the GParted LiveCD (with the USB drive plugged in!). In the GParted tool (which should show up after boot * ) you must select the USB drive (there's a drop down list showing all your hard drives in the top right of the GParted window; the external drive will be named "/dev/sdX" where X is a single letter) - if you've got more than one possible drive you should just select them one after the other; GParted will then show you the partitions that exist on each device you select, including their total size and used capacity. This should be sufficient to find youe external hard drive! **
Once you've found the correct drive, right-clich the NTFS partition and choose "delete" from the menu. Then right-click the free space and create a new FAT32 partition in the size you wish. Click the "apply" button in the GParted icon bar, and wait until you get a success message.
Then close tzhe GParted application and reboot the machine using the icon on the desktop. Boot up into Windows or whatever you use, and copy your data back on the external drive - and you're done!
*) If the GParted application dies not show up, but you have a graphical, mouse-operated desktop before you, you can start it using the GParted icon at the top of the screen.
If you're dropped to a command line instead, try "sudo Forcevideo" (commands are case-sensitive - so type them exactly!); this should give you the graphical desktop.
**) If you're not 1000% sure you've found the right drive, let us know - there are other (a bit more complicated) ways to definitely find the right one. However, on most computers, equipped with a "reasonable" number of hard drives, looking at the partitions and their sizes should be enough.