Cheers whiteb - tried that. Removed ~/Library/Virtualbox.xml, everything within the Machines subdirectory. Importing stlll gave the "2 medium" error.
Manually attaching caused the DMA error upon boot. I wonder if it's due to where the drive is connected on the controller - my hard disk image is on primary slave. What's yours? (I've tried primary master too...)
Downloaded the Live version... Will play later