Other Help Topics :: Backup without compression ?

Doing sync is a good idea after writing any file to disk when there is a risk that the drive might get disconnected shortly after, or when you otherwise want previous writes to be finished - otherwise data might still be sitting in the buffer and never have time to get written.

Especially important with slow-writing devices like usb sticks.

umount is supposed to call sync, so the buffer should get flushed on umounting the drive.

i forgot to mention that this method is only good for target media which has a linux partition e.g ext2.
FAT16/32 partitions do not preserve ownerships nor permissions.

original here.