NTFS is windows based hdd format..
Mac uses HSF format..
Windows =
NTFS (Read & Write)
HSF (No Read & No Write)
OS X =
HSF (Read & Write)
NTFS (Read)
use FAT format if you want to share files between windows and macs (FAT max size per file is 4gb)
btw, isn't a usb flash disk usually uses FAT format??