I'm kind of hoping someone who's dealt with something like this on these devices will chime in, I've run into errors that resemble this in the past but they were all scenarios that I doubt would apply in this case.
one thing you probably want to try is running an xfs_check and see if the filesystem reports as healthy, I beleive certain types of filesystem issue manifest as filefull rather than io errors but I don't know any details.
specifically, my first drive with 4k sectors did not play well with the motherboard I had at the time which resulted in it reporting 3TB of space but in practice it could only access 750GB (I think). Sadly i didn't notice until I started getting weird behavior when it tried to write past that part of the drive.
The other was when I tried creating an 18TB raid array on a 32-bit arm device (lsqvl). Similarly it would let me create the volume and even format it but any attempts to access blocks past the 16TB mark would fail.
I wouldn't think either of these would apply to your situation, particularly if these are the original disks that came with the device. But if it's happening when you try to write things from the OS as root and not just over samba, that narrows it down a bit at least.