it may sound like that...but I haven't.
i have three VMs open, but
disk/cartridge > d1: > d1:...
that second d1: is definitely greyed out
My guess is that you are in tiled mode, and the cursor is not on any of the three VMs when you choose the menu, so it does not know which VM you want to work with. You have to go into full screen mode, or have the cursor on a specific VM when you choose that menu item using keystrokes, or it can't know which of the three you intend to mount a disk in.