Upon upgrading to 3.2.8 and re-starting a client you might get an error message such as:
“differencing image not associated with parent”
“differencing but not associated with any parent in media registry”
“Parent medium with UUID x not found in media registry”

For some reason, inaccurate info ended up in your vdi file. VB is now more strict, and this inaccurate information just needs to be expunged to prevent VB from panicing. See this ticket for details:

http://www.virtualbox.org/ticket/7289

The short of it is: You download the tool for your host platform:

http://www.virtualbox.org/download/VBoxFixHdd/

“Identify the affected images. You can either use the VirtualBox GUI and check which base images are inaccessible, but shouldn’t be. The same can be done with “VBoxManage list hdds”, in this case make sure you only look at images which say “Parent UUID: base”.
Run the repair tool:
VBoxFixHdd –filename /path/to/image.vdi –zeroparentuuid
Repeat for all affected images.”

Worked like a charm.