Allocation unit size...

Which Allocation Unit Size Works Best?
(From a makeuseof article by Tina Sieber)

Hard drives are organized in (allocation units) called clusters and the allocation unit size is the size of a single cluster.
The file system records the state of each cluster, i.e. free or occupied. Once a file or a portion of a file is written to a cluster, the cluster is occupied, regardless of whether or not there is still space. Hence, larger clusters can lead to more wasted or slack space. With smaller clusters, however, the drive becomes slower as each file is broken up into smaller pieces and it takes much longer to draw them all together when the file is accessed.
So the optimal allocation unit size depends on what you want to do with your USB drive. If you want to store large files on that drive, a large cluster size is better as the drive will be faster. If, however, you want to store small files or run programs off your flash drive, a smaller cluster size will help preserve space.

Rule of thumb: large drive and/or large files = large allocation unit size

For a 500 MB USB flash drive, rather select 512 bytes (FAT32) or 32 kilobytes (FAT). On a 1 TB external hard drive select 64 kilobytes (NTFS).