You mean you want to install from a physical DVD that is inserted on a DVD reader physically connected to the server? It has been a long time since I do it this way and now I don't remember well if in this case the link to that physical device must be unique between VM. Is the other VM you already have also linked to the host DVD device? Maybe that's the problem? Not sure, sorry
Anyways, I would recommend to upload the ISO file into the datastore and install from it. And once finished, let the ISO file on it's own ISOs folder in the datastore. I use that to claim that space as the last ressort lifeline. In case some trouble would cause you to run out of datastore space (I've see sometimes some snapshots that go rogue and nobody noticed in time), to have something you could delete without hesitation to free a couple of precious GB it's priceless. And meanwhile, if not needed, to lose those couple of GB shouldn't be a problem at all. If it would be a problem not to have such small amount of GB for the daily basis, then it means the datastore should be bigger or some contents should be moved somewhere else.
Regards
