When it comes to file copying, not all X-Box programs are as robust as they could be... Usually direct file transfers are ok (for eg, off a CD), but from USB devices or via FTP services things aren't always so reliable.
(XBMC's FTP server is the only one I really trust these days, used to use UnleashX but that would also fail with certain specific files).
Perhaps in future you could try doing the update via XBMC's emergency recovery console (hold white + Y when it loads). Don't try it while XBMC is running normally however, you'll find some files become read only in that state.
You could also see if there's an update available for Avalaunch, or try an entirely different dashboard-type app.