I cannot do bootable usb stick in Ubuntu 15.04 with "startup disk creator". After copying files it says: "system policy prevents installing the bootloader". This is odd because I was able to do this somehow in Ubuntu 14.04.
It's a bug. An incredible one....
In the release notes of 15.10 beta, you can read:
Due to changes in syslinux, it is not currently possible to use usb-creator from 14.04 and earlier releases to write USB images for 15.04; we believe that it is also not possible to use usb-creator from a 15.04 system to write USB images for earlier releases. For now the workaround is to use a matching release of Ubuntu to write the images, but we intend to issue updates soon to work around this incompatibility.
See the bug here: https://bugs.launchpad.net/bugs/1325801
So it seems that, for the time being, you need 15.04 or later to build a bootable USB for 15.04 or later. Catch 22...
So to work around this issue now: create an old-fashioned DVD or
try to use the
mkusb method explained in https://help.ubuntu.com/community/Installation/FromUSBStick .