(source)Third-party apps must go through the Storage Access Framework to interact with files on portable storage; direct access is explicitly blocked for privacy and security reasons.
Is there any way to manually unblock it? Can it be unblocked for specific apps or only for all apps?
I don't run any untrusted software and I don't plug in untrusted devices anyway. I don't put sensitive data on portable storage (USB, MicroSD) anyway, so having direct access is no privacy/security risk to me.
To me this just sounds like fearmongering to get users into accepting these restrictions that make your phone far less practical. It is the same type of "privacy" and "security" that you get when disconnecting your Internet. Sure, you are more secure, but you also accomplish far less.
Imagine Microsoft restricted USB stick access on Windows like this "for security reasons". Yes, more locked down can add security, but it comes at a severe cost of usefulness and freedom. Should something not exist just because it can be misused? With this argument, one could also ban cutlery, because a possibility of misuse exists.
Google needs to stop taking freedom away and excuse it with security.