Error while attempting to take a snapshot [SOLVED]

[rebelxt]

I am trying to take a snapshot of a Fedora 12 Live CD VM running on a WinXP host. I get an error message:

Failed to create a snapshot of the virtual machine ...
Medium 'D:\...' is not locked for writing.
Details:
Result Code: VBOX_E_INVALID_OBJECT_STATE (0x80BB0007)

Help?
Is there something I need to do?

Edit: I discovered what the problem was. The virtual hard drive attached to this VM was also attached to another VM. I attached a different, otherwise unattached, hard drive, and the snapshot was successful.