I started up my VirtualBox this morning and was amazed that it told me that the DVD ISO image I have still mounted was no longer accessible.
So what has happened since last night? Well - I charged my iPod on my laptop before I hooked up my external USB drive which contains the ISO image. Apparently Windows changed the order of the drive letters (was E: is F: now).
So after all it seems that VirtualBox has not so much a quirk as it connot do anything better that accept what Windows will feed into it.