One of the features in Android 6.0 Marshmallow that hasn’t gained many headlines is called flex storage or adoptable storage (Google uses both terms).

The new feature will allow you to use external storage on your device as internal storage. It’s likely to be popular, as you’ll be able to buy a more affordable device with low internal storage and boost it cheaply by adding amicroSD card.
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https://www.pocket-lint.com/apps/news/google/135716-how-to-use-adoptable-storage-on-android-6-0-marshmallow