If I might preface this by saying that I have learned my lesson!
I forward port 23 to the outside world so that I can fool around with my mybook from other locations. Though I had changed the root password, it seems my choice was not strong enough, because whilst browsing around my shares last night I observed several strange files, one of which was a zip containing a password dictionary. Log files revealed a dictionary hack coming from 184.108.40.206 at the Universidad de Extremadura.
At first I feared it was bricked, but amongst the steps I have taken to clean up was to perform a long overdue firmware update (automated). Since doing so ordinarily clears out all changes made (optware etc), can I be reasonably sure that nothing of the infiltration remains behind? (indeed all of my own changes for sabnzbd and so forth have all gone now)
Thank you all!