Hmm. Despite efforts to the contrary, it seems I've managed to explain myself badly, and come across rather arrogantly at the same time. My apologies.

I don't think that changing storage locations, whether default or actual, around is in itself misconfiguration at all; my point was that making those changes but not telling SQL Server about the amendment may well qualify, despite the scenario you highlighted having very little impact. In this, I made an (unsafe) assumption that this is what you had done, since when I've edited the registry to point to another drive as the default backup location, it's worked fine, and the backup command has picked up the new default location.

Not the first time today that what I've said and what I've meant have been different, either. I think I might head back off to bed as soon as possible....