There appears to be an error in the parsing of the relative path for CD/DVD images.
I made am VM that had a hard disk ( in .Virtubox\Hardisks) image and left the corresponding CD Iso image mounted (which I had placed in .\hardisks so the path was relative to the location of Portablebox)
This works fine Until the path changes (different machine / different drive letter / etc) at which point I get an error about not being able to find a CD/DVD with UID ......
Unfortunately at this point nothing I can find seems to recover, unless you can get it back to the 'original' path, at which point you can successfully access the VM and release the CD/DVD.
SO it looks like the path for the 'hard disk' is parsed to for the changed path name, but not CD/DVD images (and I would presume likely floppy, etc)
.
Though thanks for a very useful product.