Time to clear the air on a couple adoptable storage myths propagated mostly by Samsung and LG as excuses for why the feature is absent from their ROMs as shipped.
I have 2 devices with adoptable storage enabled: an Nvidia Shield K1 running stock Shield software and a Samsung SM-G900V (Verizon Samsung Galaxy S5) running CM 13.0 nightly. Both OSes are based on Android 6.0.1. The feature works perfectly on both with no slowdowns or delays.
Sadly, much of the confusion about the feature stems from Google’s poor communication about it to begin with. The fact that the most extensive technical writeup on the feature is a Reddit post – some of which I disagree with based on my own experience – is quite damning.
Let’s start with what adoptable storage actually does. The best analog of adoptable storage in its default implementation is moving your personal files to a slower hard drive on your PC while keeping your apps on a fast SSD. That’s what adoptable storage does by default. It just changes the location of your personal files to the microSD card and reserves your phone’s eMMC or UFS storage for apps.
Myth: Adoptable storage slows your phone down.
Relative to keeping all your data on internal storage, this is correct. But when flagships ship with only 32 GB of storage in 2016 when even $600 laptops ship with 1TB, the assumption that users would only ever use the phone’s internal storage is anachronistic at best.
Once the portable storage use case enters the picture, the speed argument falls flat because both portable storage and adoptable storage fetch user data from microSD while running apps on internal storage. Oh, and local storage will always be faster, more accessible, and more reliable than cloud storage, so comparing adoptable storage to pulling files from the cloud is also nonsensical.
Significant slowdowns happen ONLY if you move your apps to the SD card also, which doesn’t happen by default on the devices on which I have it enabled. Or at least, it doesn’t seem to happen on devices with >= 16 GB of built-in storage (Then again, implementation seems to have been left up to ROM developers). Unless you run TouchWiz or play a lot of games, Android apps are very space efficient. Here’s the storage on my S5 runniung CM 13 with 101 apps (not counting the Open GApps package) installed from the Play Store:
Myth: You lose access to your internal storage.
Wrong. Rooted users can still read and write to internal storage using apps like Root Explorer, but other non-privileged apps won’t be able to do the same.
Myth: You need portable storage to transfer files to PCs, and adoptable storage ruins that.
How, in 2016, “experts” are recommending sneakernet for file transfer is beyond me. Popping a microSD card out of your phone to put files onto a PC makes as much sense as pulling the PC’s user data hard drive to transfer files onto another PC.
The easiest way to transfer files to a PC from a phone are apps like Send Anywhere, FTP, SAMBA, or BitTorrent Sync, or hardware features such as USB OTG. There’s absolutely zero reason to insert your microSD card into your PC, especially since doing so can cause other problems.
Myth: There’s no advantage to adoptable storage over portable storage.
The only real disadvantage is fast(er) eMMC/UFS internal storage is no longer accessible for personal data use, but this is relevant only to users who have very small media libraries. Once your library exceeds the paltry free storage available on current 32 GB flagships, adoptable storage is the best route.
Adoptable storage gives the rest of us with GBs of documents and media a gigantic (relative to internal) block of storage that any app can read or write to. Compare that to portable storage, where App A can’t modify App B’s microSD files. It also blows cloud storage out of the water in terms of speed, availability, subscription cost, and mobile data usage.
Myth: microSD cards are unreliable.
Adoptable storage SD cards aren’t any more or less reliable than portable storage ones. You get what you pay for. Cheap, off-brand, and low spec cards will be awful. Ensure you’re using U3 rated card at the very least. I recommend Samsung, SanDisk, and Kingston cards, in that order.
I’ve had a great experience with the Samsung Micro SD Pro+ 128GB. Get it.
/jdrch 
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