updating from sierra to Mojave using external Samsung T5 ssd ( 2 T5 500s 1boot, 1backup / sierra , one 1Tb back up and imac hd) on late 2015 imac 21.5...... I download mojave from app store get message; "no guid partition click volume scheme, erase" I do "volume unmounted" mount option is grey so I ccc from boot to erased disk guid partition in place get "must be 16 -30 gb space not enough room" there is 401 gb available > I do first aid and get "volume unmounted".......its a whos on first....all I want to do upgrade to mojave thanks
