Dual Boot with Grub doesn't work

Hi,

used this howto

Dual Boot - OpenELEC

to set up dual boot on my htpc. my libreelec v8 is installed on a usb drive and i installed ubuntu 16.04 on the same drive (/dev/sda3). for that test i didn't create a swap partition. but after reboot there isn't a grub menu. ubuntu starts. i have so chance to start libreelec. i also tested this with an other usb drive with an old openelec v4 with the same result. isn't this working anymore?

this is the file /etc/grub.d/40_custom that i used to create a grub menu for openelec v4. the same with libreelec.

menuentry "OpenELEC" {    
    search --set=root --label System --hint hd0,msdos1
    linux /KERNEL boot=LABEL=System disk=LABEL=Storage quiet
 }

i also tried uuid:

menuentry "OpenELEC.tv" {    
    search --no-floppy --fs-uuid --set=root 21247e93-512c-4aab-894b-caa7cb3a8930    
    linux /KERNEL boot=UUID=21247e93-512c-4aab-894b-caa7cb3a8930 disk=UUID=df911cc6-741b-4f8a-8ef1-3ca012a95d58 quiet
}

this also doesn't work.

whats wrong with that?