You might think that buying a handset with 32 GB of internal memory and adding a 32 GB microSD card to is the same as buying a 64 GB smartphone. It's not.
This important. It is always cheaper to add external storage, and thus tempting to do so, but it’s just not as effective. Why? To explain this we must look at life before and after Marshmallow.
Before Android 6.0 Marshmallow
On devices that don't run Marshmallow, the kind of data that can be stored on the microSD card is different to what can be stored in your phone. You can store movies, music and image files easily, but app data (including sometimes large game files) is trickier.
You can’t, for example, install an entire app or game onto a microSD card. You can move part of the data there, but that's all.
Because of this, even with a 128 GB memory card stuffed into a 16 GB device, you could still find yourself frequently running out of storage space. So, first and foremost, microSD cards are more limited in terms of the type of data they can store, while internal storage can hold any data.
MicroSD cards can be used for good.
After Android 6.0
Google has made an attempt to unify the MicroSD / internal storage experience in Android Marshmallow with a feature known as Adoptable Storage. With this, you can use a microSD card just like internal storage. The caveat is that once you do this the microSD card can only be used in that device, and removing it can cause all kinds of problems.
moto g 2014 marshmallow cm13
Devices with a microSD card slot will be able to format an SD card as internal storage.
Though it may act the same as your device's internal storage, it's still not quite the same.
The speed at which your device reads and writes information to storage mediums affects overall performance.
Traditionally, accessing data on external storage mediums was almost always slower than the dedicated drive. With Adoptable Storage, this is still mostly true: the unification of these two storage mediums will have an adverse affect on overall read and write speed.
But to make the situation even more complicated, performance may improve in rare cases.
On the HTC One A9 and confusion
With the HTC One A9, a MicroSD card can actually improve the phone’s read and write speeds. It depends on the type of SD Card, but when using the SanDisk Extreme Plus 64 GB in our One A9, the overall read and write speed actually increased. Truthfully, it was not to any visible degree, but worth mentioning all the same.
androidpit HTC One A9 Format internal storage
Using a high-performing microSD card in the One A9 is probably a good thing.
On the Galaxy S7 and confusion
The Galaxy S7 has made matters worse because its MicroSD cards don’t make use of the Adoptable Storage feature, despite the fact that the S7 runs Marshmallow.
Based on current information, it seems that SD cards in the Galaxy S7 and Galaxy S7 Edge will act the same way as external storage does in the traditional Android sense, that is, they can only store photos, videos, app data, and other such files.
Where does this leave us?
Android Marshmallow has attempted to bring the warring factions of internal and external storage together. There is currently a ceasefire: these storage forms are playing nice. If you take advantage of Adoptable Storage, your MicroSD card will act just like internal storage with few downsides. But some families, such as LG and Samsung, haven’t implemented Adoptable Storage.
For now, I’d still advise against buying additional storage if you want the same function as internal storage, unless you know your device supports Adoptable Storage and don’t mind a potentially adverse effect on performance. Additionally, external storage mediums are prone to corruption, and who knows what the long-term effects of persistent data transfer will be when microSD cards are used like internal storage.
If you just want another way to store pictures and videos without clogging up your internal memory, a microSD card will still do just fine.
