Quoting Patrick Shirkey <pshirkey(a)boosthardware.com>om>:
For example if I transfer 200mb to a usb disk the copy
command takes
about 30 seconds before it returns and the data takes about 5 minutes
before it is actually finished being transferred and the device is
unmountable.
quick-hack(tm)
(cp -a /xxx /usb_device && sync) &
But remember sync flushes _everything_.
Sampo