These are the default cluster sizes for NTFS
512MB or less 512 bytes
513MB to 1024MB(1GB) 1024 bytes (1KB)
1025MB to 2048MB(2GB) 2048 bytes (2KB)
2049MB and larger 4096 bytes (4KB)
As you can see a 15GB drive will have a 4KB cluster.
This would seem to indicate that the partition with 512b clusters is not NTFS or that during formatting the defualts were changed by the user.
Can you confirm catagorically that the partition is NTFS.