If it's really free-space, then you would use fdisk to create a new partition, and mkfs.vfat to format it. Check the man page on both before trying anything.
The basics are... fdisk /dev/hdx . Change the hda to whichever disk it is...
primary master = hda
primary slave = hdb
secondary master = hdc
secondary slave = hdd
In fdisk, you can use m to print a menu of command, p to print the current partition table, n to create a new partition, t to change the partition type, l to list the acceptable types, and w to write the new table and exit.
Then, mkfs.vfat -F 32 /dev/hdx# to format the new partition. Be sure to use the correct drive / partition designation. The hdx is the same as above, the # is the partition number you created. You can see all the partitions with fdisk -l . You have to specify the -F 32 to get fat32, otherwise it will default to either 12 or 16.