Fat32 partitions are not compatible by default with Ubuntu (except for the new one, Gutsy Gibbon or 7.10 - this can write to all Windows file systems). If you just want it to put files on, format it to and ext-3 file system. If this doesn't help, login as root and in the terminal change directory to the mount point for your hard drive, and change the permissions using
chmod 777 /mnt/hda1 (or whatever your drive is, and 777 makes it accessible to everybody, so change the numbers according to what you want - they are octal)
-Stephen