But anyway, you don't have to hack anything, they literally give you all the access you could ask for. Run samba on it if you want.
* I'm sure there are exceptions, but they're not common in my experience.
I have no issues mounting Linux fs, since my OS...
> have two devices mounting one disk at the same time
There is exactly NO reasons to allow mounting when the device OS is shut down and there is non reasons to not allowing mounting a data only volume who can be balanced read-only on one or another side. It's simply an arbitrary choice.
Anyway, as far I know their OS, a custom GNU/Linux distro is open in the sense of the license but I even need to host their own "fake cloud" just if I want a no-someone-else service needed setup and the unofficial repo is not much more exiting https://toltec-dev.org/stable/
Being open to me does not means "hey you have the code" like AOSP, is being damn open EASILY, the device is mine, I handle it as I handle a desktop computer.