I was running into the same issue after migrating to a new storage system the vm had still listed both storage systems in use. I thought maybe it was still connected to an ISO image on the old storage but that was not the case either.
I then tried to mount an ISO image to the VM and the dialog box first pointed at the old storage system. I switched it to an ISO on the new storage system which seems to have severed the connection to the old storage system. Now the old storage system is no longer listed for the VM.