About

What is "adoptable storage"?

Adoptable Storage was introduced with Android Marshmallow (6.0-marshmallow) to address the permanent issues with too low internal storage (especially on budget devices — see insufficient-memory) by "adopting" external storage to substitute for the internal storage.

If your device supports external SD cards and runs on Android 6.0 or higher, on insertion of a card you will be asked whether to use it as "portable storage" (i.e., the same way you've used it before), or if Android should adopt it as "internal storage":

^{Screenshot (click image for larger variant)}

What happens behind the scenes?

If you chose to "adopt" external storage as internal storage, it will be encrypted. Which means multiple things:

• this storage can only be used in the device it was "encrypted into"¹
• you no longer can simply switch it to another device: lacking the encryption key, it will be unreadable there
• you no longer can use a card reader to access the card's content in case your battery run out (or your device gets damaged), to rescue some data
• no "simple upgrade" to another (bigger, healthier, faster) SD card
• all data on the card will be erased while "initializing the adoption". Same applies if you want to revert the process.
• whatever real internal storage you had will become invisible to both mtp and normal (non-root) file management apps, and even then, it can only be accessed with considerable difficulty as a mounted storage.
• when you factory-reset your device, you cannot even use recovery software to rescue data from the card (as the encryption key will be gone as well).

Further readings:

• How Android 6.0 format as internal storage feature works?
• Understanding Adoptive Storage in Android 6 (Marshmallow)
• Adoptable storage in Android 6.0: What it is, how it works
• How to decrypt adopted storage?
• Corrupt SD card formatted as internal storage (answer is a tutorial on how to mount that card on your Linux PC)
• How to split adopted storage? (multiple partitions)
• How can I move data from an adopted SD card to a new SD card?
• How to adopt SD card as internal storage on Samsung devices?
• How to get Adoptable storage encryption key without root access after I “forget it”

^{1: Basically. There are ways around this. See Corrupt SD card formatted as internal storage or Decrypting Android M adopted storage for examples}