The Android M Developer Preview 2 has been out for more than a week now, and there have been a number of new features and tweaks which have been discussed to showcase what loading this early version of the software would bring to a device compatible with it. Among these were things like the System UI Tuner which allows users to customize what icons they see in the status bar, as well as some improvements to the app drawer including a new landscape mode, and improvements to the memory. As of today some more changes are begin discovered which shed some light on what we might be able to expect when Android M is finally made official and starts heading out to devices sometime later on this Fall.
One of those new functions today happens to be a new permission required for applications. More specifically, apps will now require permission from the user if they want the capability to read and write to external storage in the phone. This is alongside the other permissions changes with Android M which lets users grant or refuse permissions to apps when the permissions are actually needed as opposed to immediately at the app install process. This new tweak as part of the dev preview 2 is perhaps a means of cutting down on potentially risky behavior, as Google has deemed it so that it is dangerous territory for apps to have access to storage outside their own personal file set. Meaning, if an app wants access to read or write to any external storage where other apps also already have access, it needs permission, and you'll be prompted to allow it when the situation comes up.
In the release notes which describes all the changes to the Android M Dev Preview thus far, Google explains that these permissions prompts will not be limited to external storage, and that internal storage may sometimes require permissions as well if an app is requesting read or write access outside of their own specific file folders within the internal storage too. Permissions stuff may be boring to users, but they're changes that are likely needed as the software continues to evolve. Just because these functions are here now however, does not mean Google will opt to keep them when a final release build hits the airwaves.