Best answer: Adoptable storage is when you use a regular microSD card and have it act as if it were internal (non-removable) storage on your phone. It's slower than the actual internal storage, can fail if used too much, and once you do it you can never remove the card again unless you reset your phone. That's why many Android device manufacturers don't enable the option.

What is Adoptable storage?

Android has always supported external storage and SD cards, but with Android Marshmallow we saw a new way to use them: adopt them into the system to act as non-removable storage just like the storage that is already inside your phone.

It's a good idea that has been around a long time, but there are some pretty serious drawbacks to doing it. The first thing you need to know is that once you adopt an SD card as system storage, you can't remove it unless you want to factory reset your phone. That's because system files for apps and settings now live on the card and if you turn the phone on without it, things will go haywire and never be able to recover; the system expects and needs certain files to be in a specific place.

Adoptable Storage makes your SD card non-removable. Unless you enjoy factory resets, that is.

Another drawback is that even the fastest microSD card is a lot slower than internal flash storage. You might think that the numbers you see advertised like 300 M/second read time are fast, but those numbers only apply when you're reading one big file. Smaller sequential reads of a lot of little files — which is mostly what the system will be doing when it lives on the SD card — will be much slower. Probably slow enough for you to see the difference, and maybe even too slow.

Finally, you need to know that SD cards, even the best ones, have a limited number of times they can have data written to them before they fail. The internal flash storage in your phone will fail eventually, too, but you probably won't have your phone by then as it will last hundreds of times longer than an SD card will when used as system storage.

It's not all bad