I'm trying to create a boot-time disk (+ image). I choose in the menu Disk -> Add boot disk, but the image is loaded during the login process, and not during the OS boot (this is noticeable since the login is very long).
In this case, the mount sequence had started at boot time and continued until the moment you logged in. It does not prevent the system from continuing booting or displaying a login screen.
Try booting the system and waiting 1-2 minutes before logging in: the disk should be fully ready by that stage.