What is really meant by "Invalid B-tree node size"? How to solve it without expensive 3rd party softwares?

I've been having troubles with my external HDD and have been looking so hard for a solution. I've tried everything. Diskutil, FSCK, testdisk. But only to find what I already know... "Invalid B-tree node size". It's unbelievable that Apple hasn't included a fixer for this kind of problem within its tools like diskutil. My concern is, what is the meaning of this error? Is there any solution to this problem without using a expensive 3rd party software? Isn't there any open source software to recover my data? Has anyone fixed this problem before? I certainly would love just to buy a specialized software like DiskWarrior, but I can't afford that price.