btrfs_merge_bio_hook+a0 error, recovery possible?

I feel your pain

I had exactly the same issues about a year ago with this error. I knew nothing  about linux but  in getting a solution that worked for me  had a crash course

To attempt to translate into common speak with a degree of licence for the simplistic explanation.

The actual volume presented is in fact as you have been found from a number of discs and also a number of separate volumes on the discs.

Also when you expand via X-Raid another volume is create with the free space.

The operatiing system then "meshes", hooks, these separate volumes together into  what is presented as "One Volume"

So what the error message is saying is "that it cannot find the entry/exit points on these separate storage volumes to  generate the one large volume."

The checksum errors are shown when trying to read across the boundaries but with no way of undertstanding where the data is in the volume.  (or something like that)

I delved deep into this and found a range of articles on how to restore  a btfrs volume After 4 weeks of tinkering I gave up.  I contacted a number of commercial companies and was told on more than one occasion that "recovery  cannot be done who would use btfrs in a commercial system anyway.!!"  Others gave a very high initial charge plus  an even higher charge to recover if it was deemed viable.

Anyway I fell back on an old colleague of mine who was well versed in linux and actually he put me on to a "Windows" solution  One was ReclaiMe and the other was UFS Explorer Pro  I tried the trials and found UFS Explorer  saw all my data.  In fact my friend had a colleague who had a copy so I sent my discs to him and he recovered my data.

Job done Then  I came across this article. A similar problem (different NAS supplier) but actually the same underlying explanation that I found out by my amateur investigations and analysis.  Google " anandtech nas recovery"

As for linux  before getting to my solution I tried a lot of Linux programs I would hestitate to say all but tried most of the "forensic tools"  BTRFS support is poor.  What stopped me from a factory reset was when I used PHOTOREC this recovered a multude of  "sections" of my data.  This indicated that my data still existed it was getting it off that was the issue   There is still a lack of Linux recovery programs   when  proprietary expanasion modes are used even though the basic linux commands create these volumes.

Anyway try the Windows solutions  They worked in my case