We've been exploring all the goodies in Android Marshmallow with the Nexus line, but there's one tantalizing aspect of Marshmallow that couldn't be explored until the HTC One A9 came along: adoptable storage. Unlike the Nexus devices, the A9 has a microSD card slot, and while traditionally a microSD could take some of the burden off your internal storage, there were a lot of caveats. Even though there may be dozens of free gigabytes on your card, you'd be out of room on your internal storage and out of luck.
In Settings, we come down to Storage and USB. Your can select the card you insert under Removable storage. If you haven't already, you should ensure that everything on the card in question has been backed up. We're going to tap the menu in the top-right corner and select Settings. We have a few options here.
Eject will un-mount your card for safe removal. Format will erase the card and re-mount it as a regular removable storage. Format as internal will wipe your microSD card, encrypt it, and set it up as internal storage; that's the one we want. Formatting as internal doesn't take long, but offloading content from internal to the newly formatted microSD can depending on how much data needs to move.
After selecting format as internal, we get a nice little explanation and a warning that this will erase the card. The Erase and format button is also a nice warning orange, and we get another warning pop-up. Also, as we are encrypting the card, you will not be able to simply pull the microSD card out and stick it back in your computer or another phone without losing all the data.
You have been warned. Now, tap format and let the system erase, encrypt, and format your card. Grab yourself a cold beverage and think of all the extra movies and games you'll be able to download now.
When the formatting process is complete, you'll be given the option of taking an extra step: actually transferring some of your data to the card and freeing up that space on the internal storage. This can be a time consuming process, and while you can keep using the device, you should just leave it alone during the transfer. As the screen states, some apps might not work properly during the transfer, and that includes some system apps/features.
And now, you're ready to pin a marathon of movies, or download a gaggle of games, or take a heap of HyperLapses. And we cannot wait for this to hit more devices, because it has worked fabulously.