I am installing Ubuntu 15.10 (final) and I encountered this issue (exactly as described by the original poster). This issue is mentioned in https://wiki.ubuntu.com/WilyWerewolf/ReleaseNotes as well, in the Known Issues section.
However, in my case the Force UEFI dialog cannot be closed, thus, it is not possible to continue the installation. -- You received this bug notification because you are a member of Ubuntu Bugs, which is subscribed to Ubuntu. https://bugs.launchpad.net/bugs/1447256 Title: "Force UEFI" dialog displayed if user selects custom partitioning To manage notifications about this bug go to: https://bugs.launchpad.net/hundredpapercuts/+bug/1447256/+subscriptions -- ubuntu-bugs mailing list ubuntu-bugs@lists.ubuntu.com https://lists.ubuntu.com/mailman/listinfo/ubuntu-bugs
