This is not really a question for the General support section, mainly because dual boot stuff is outside the scope of LibreELEC.
Seeing the LibreELEC system partition is the sixth partition on your SSD, what else is installed on there? Did you create the LibreELEC storage EXT3 partition manually? Not EXT4? I can think of reasons to use EXT3, but I think the differences for LibreELEC are trivial.
First you state that 0,5=boot and 0,6=storage, later it's boot=sda6 disk=sda7...?
Personally I use Ubuntu Linux and use GRUB to do the LibreELEC booting from a separate SSD. LibreELEC should support GPT afaik, but sometimes some machines are simply allergic to LE. Or vice versa.