Feasibility of Guts using the Dragonslayer In the Berserk manga, given the enormous amount of chapters dealing with Guts'  over-sized sword, the Dragonslayer, I didn't try finding a pic that clearly shows the sword hanging slantingly along his dorsum. Neither did I notice the following issue before completing all the available episodes.

I'm intrigued by how exactly Guts was always able to wield Dragonslayer without at first having any difficulty from the hooks holding it, unhinges it to be more precise to say. 
If we assume having his arm fully stretched upward and trying to displace Dragonslayer from its stationary position, then prima facie the distance between his grip (not of sword's) and shoulders cannot be greater than or equal to the length of the blade of the Dragonslayer. 

It is this impracticability that made me to request here, a picture of Guts along with the Dragonslayer featuring his dorsum without any cape, so as to understand the truth. While a picture will speak the answer by itself, a context around it or without the picture itself may help too.

Edit: 
On a side note, these pictures of Guts somewhat shows the hinge which holds off Guts' sword. But I can't fathom how then he could use the sword instantly when the sword needs to be unhooked every time to slay the enemy. 

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(Click image to enlarge it)
So if I am understanding the question correctly, you are wondering why he has no issues with unhinging the sword from his back and getting it into battle position.

Let me instantly break the bad news, in real life this would not have been possible. And it  falls under the BFS trope.

Even so people tend to want to investigate how 'impossible' it would be to wield such a weapon. And which real life weapon would be most comparable to this. They came to the following numbers:


Guts is roughly 1.90 meters high
The Dragon slayer is roughly the same height (the size is a bit variable through out the series)
The blade most likely weights about 220 pounds or 99.8KG. Although on the wiki, it has been stated that it should be around 400 pounds, and some reddit user calculated it to be around 625 pounds or 283.5KG


So coming back to your question, to displace the sword, assuming that Guts and the blade are equal in height and the blade weighs 220 pounds. Trying to pull it with a stretched arm length of 0.95m (it is said that the length of fingertip to fingertip is roughly your body length) 

(not sure how to do the maths from here, but feel free to edit it in)

And to round up the post, the closest resembling sword to that of Guts', would be a two handed ceremonial sword which only resembles the blade in length (199cm / 78.3inch) and weighing much less (5.42KG / 11.9 pounds)