So, I've had a ton of issues with the SD card unmounting itself of its own accord. I thought it was from an app (Delayed Lock) I installed, since it seemed to start after that, but I uninstalled it, and it was still doing it. Mostly whenever I plugged it in to charge it, tried to update any app on the Play Store, or tried to use any file exploring app, like ES Explorer. It would also just "forget" the SD card randomly, so any app on the card would say it was not installed, or inaccessible, and the phone would restart itself sometimes, after forgetting the card. After removing that app, it wouldn't restart itself, but was still just forgetting the memory card, saying I had no card installed. I had the SanDisk Ultra in there (Class 10/U 1), recommended for the HTC 10.
It did work fine initially, as adopted storage, but, once the issues began, they have been steady, at least ten times a day, and incredibly frustrating, since I could not access important apps without restarting the phone, or even having to physically remove and re-insert the card, sometimes. Other times, within a minute or less, it would "remember" the card, no restart was needed, and I'd see the apps return to the screen, although I'd have to then go and re-set my ringtones/notifications each time, since those were on the SD card and most times the settings did not return, even once the card re-mounted itself.
What really sucks about the adopted storage idea, in trying to fix this problem, is that the data ONLY is accessible, within the phone, so, even trying to access photos or music on my computer, the card on its own isn't even recognized and can't be accessed, unless still in the phone. I was able to move most of the apps back, save the photos and music while the card was still in the phone, but then I couldn't access the phone via USB on my computer, since it still was associating internal memory with the original card I had in there. Even another card didn't solve that. I had to put in the original card, and re-format it as portable storage, to "release" the internal memory of the phone. I had wanted to save the card, in case I needed to transfer anything else, but the only solution was reformatting it, and then I could access the phone again, but, by then, I lost what I lost, as I am slowly finding out now, noting apps that aren't working, games that had 200+ levels of play, now back at level 1, ringtones and notification tones missing, etc. Small annoyances, and not a big deal, but irritating nonetheless.
So, I now have a new U1 speed card in there, a PNY Ultra, temporarily, and have ordered a SanDisk U3. However, I will no longer use the adopted storage. The only reason I did was added security, since the SD card gets encrypted along with the phone, but apparently the SD card can be encrypted as portable storage as well, then decrypted if you ever wanted to access it, independent of being in the phone, so that seems a good way to go. I don't have anything on my phone of much security concern anyway, so I think, based on the hassles of the adopted storage, I won't bother using that set up again, and either use the local card encryption or not bother to encrypt it at all, and anything like financials or whatever, I will save to the phone, since it is encrypted already.
All in all, a bad experience with the adopted storage. Not sure if that means I got a defective phone (I'm on T Mobile, but bought it, unlocked, direct from HTC) or they just have not ironed out all the kinks in this adopted storage concept, but the glitches are not worth the frustration, since it really doesn't gain much, plus shortens the life of the SD card, which you then run the risk of not being able to even access, if it starts going bad on you.
I'll see how the phone works just using portable storage, and hopefully it will work well -- this is my first frustration with an HTC phone, coming from the original MyTouch Slide, to the M7, the M8, skipping the bad sequels and waiting and waiting for this one, so I hope it settles in and works well. I hate having to replace a phone and start from scratch. Other than that, however, this HTC 10 is the best tech device I've ever owned -- the sound quality blew my mind, lol, I've never had any device be able to produce such high quality, plus the camera, video quality, screen quality, etc., so, yes, I hope this solves it, since I really want to be happy with this new HTC gem.