If you have an existing .hdd can you attach it / mount it to a running VM ? I know you can shut down the VM and then config -> hardware and + in a HDD "existing image file". The question is whether it can be done without the shutdown? I experimented with using Devices -> CD/DVD -> connect image, but that is locked to finding .iso and .dmg files . I experimented with using Devices -> CD/DVD -> configure -> CD/DVD -> Source -> choose an image file . With that I could using the options section to select *.* and then I could (not reliably) sometimes select a .hdd . However, Finder then reports that it is a blank disk.. I guess because it does not have the ISO header that would be expected for a DVD.
