Android M will finally allow users to move app data and APKs to microSD cards
The new feature is called Adoptable Storage Devices. Although the company didn't talk about this new feature yesterday on the Google I/O stage, references to such a feature can be found in the online documentation for the Android M developer preview. Adoptable Storage Devices will allow the user to move both the app data and the APKs to the external storage, but only if the app developers specifically allows this in the Android app's manifest.
The official documentation says that adopted storage devices will be encrypted and formatted to behave like the internal storage. Since the microSD card will be encrypted by the phone, other devices will not be able to see the data on the card. One question that arises is how Android M will manage the scenario in which the user pulls out the microSD card slot. Most likely, the app will no longer be functionable until it is reinstalled on the internal storage or a new microSD card that has been adopted by the phone.
In an interesting twist, Android M's Adoptable Storage Devices feature will also be applicable to external USB sticks. Android M will come with built-in support for USB OTG, but chances are that you're not going to keep an USB stick permanently connected to a smartphone or a tablet. On the other hand, this could be a most welcomed feature for upcoming Android TVs or Android game consoles.