I've never really understood the whole concept of the back scabbard. I guess if you're a thief or a ninja who is scaling walls and what-not, then it might make sense, but a warrior type who needs to be able to draw quickly? It just seems like it would cost you valuable seconds.
The bigger problem is, for most swords and certainly for these two we're talking about, you literally cannot draw the sword from your back. You'd have to remove the scabbard and then draw the sword. So yeah, I guess if you need to climb something fairly sheer, you'd want to attach it to your back to get it out of the way. But that's it.
Okay, I'm gonna need to see video of a dude waist-drawing a sword as long he is tall to believe it. There's gonna be something different about that scabbard to allow it, imho.
To be sword-nerdy for a moment; in the toy form we have it, Stormbringer is too long to be drawn from -anywhere-. Swords of that size aren't worn. Period.
Could you? Yeah. It's actually not that difficult. The blade only actually reaches the underside of his breastbone, just about. You don't have to factor in the length of the hilt when it comes to how difficult it is to draw from the scabbard. So if you make sure the throat of the scabbard is fairly low/far back (accomplished by wearing the sword at a shallow angle to keep it from dragging on the ground behind you - which IS how many longswords were worn), you can certainly get the sword out. Especially if you follow the tradition of some curved swords and cut a notch out of the upward-facing edge of the scabbard, allowing the blade to come free without -completely- clearing the scabbard throat first.
The bigger problem is how WILDLY impractical it is to wear a sword around that long. Even longswords and rapiers, in historical writings, were complained about because they're such a pain in the ass, tripping people behind you, banging into things, knocking things over. And large swords have large hilts, which means a ton of hilt projecting in front of your body. It's just unrealistic to try to -exist- in the world.
My understanding is that back scabbards have a slit along the upper side, plus a generally wider mouth; that allows one to pull the sword forward to the front.
In fact, there's just no such thing as back scabbards. It's complete fantasy. Doesn't exist. And even in movies/TV they have to do 'movie magic' to get the weapons from the scabbard into the actor's hand, or vice versa.
A waist scabbard has to be drawn almost entirely outwards from the mouth as a long side-slit risks the sword falling out entirely.
While slits in the scabbard weren't -common-, they ONLY exist in waist-mounted scabbards because that is the only kind of scabbard that actually existed.
They were used for curved swords, though. Not super long swords. As above, it just gets to a point where wearing a sword of a certain length is impractical in and of itself, to the point where it doesn't even -matter- how you'd draw it.
Damien likely has better expertise
I feel called out by this.
Okay, I have to issue a retraction. On my lunch break I went back and reread the bit about the length of the Sword...it actually says Stormbringer is almost the length of Cymoril, not Elric. Since Cymoril is at least a foot shorter than Elric, I'd estimate Stormbringer to be around 5 ft. Still, a pretty long damned sword.
Tip to pommel, 5 feet is a big sword. But you're talking about a 4 foot blade. BARELY longer than many rapier blades.
And the real answer is, a sword over a certain length would realistically just be carried, either rested on the shoulder parade style or over the shoulder in it's sheath, but that would be for transport. You might have one stored on the back (again, strictly for transport), but if you were even somewhat thinking you would need to use it, you'd have it out and ready to rock long before you needed it. Having a big sword means avoiding instances where you'd need to snap draw it.
We also have to appreciate Moorcock is not a realist. He's not necessarily describing stuff that would actually work with exacting detail and accuracy.
I wrote up a bunch of my post before reading this and I am not going to delete it all but.. yah, this. The larger two-handed swords were just carried. That's it. That's the big secret.