Can confirm the app monitor is a culprit. With this and other Samsung gimping of SD card along with root prevention it will be a long 2 years till end of my contract and then farewell Samsung! Poor decisions and refusal to listen to the customers who helped get them to where thay are now = poor business strategy for the long term.
I've had similar issues to everyone else: SD card works fine for days then disappears. I just started logging the issue and I'm trying the Power Saver fix so we'll see.
But something else to look are the memory and storage optimization functions. Today I used my SD card all day on a road trip. Got home and it was still there. Ran the system cache cleaner from Storage and didn't think anything of it, but 5 minutes later when I tried to play music my card was gone. Took the card out and checked it on my computer and it was fine. Popped it in and out a few times and never got it back. Tried another couple of cards including a clone (same manufacturer, size & content) and all worked fine. Saw a trick mentioned online, booted into recover & cleared the cache. Popped the malfunctioning card back in and its working fine again.
To update a previous post: The cache wiping does not seem to work. Nothing seems to work reliably. In my case I don't have the multiple unmountings. Instead it loses the SD card and won't remount it.
I tried the Power Monitor fix. Didn't help. I tried the other fixes mentioned here and on other sites. None helped.
Yesterday it recognized my card and I used it for about 10 hours. I stopped for about an hour and when I went back to listen to more audio and the card was gone. I popped it in an out 10-20 times, while the phone was on, off or just restarted, and it would not recognize the card. I tried reformatting the card on my PC (since it wouldn't recognize it on the phone to reformat) using all easily available formats (fat, fat32, exfat, ntfs) and it would not recognize the card.
When I tried a new card it recognizes it. Then I restarted the phone and it lost the card and would not recognize it again. I tried a few more cards (I have about 2 dozen of various sizes and makes) and it recognized each on insert. Some it lost after that, some it hasn't yet but without a longer time to test I'm just waiting.
Randomly I've had it lose my main card before and after some random time, and maybe after a few attempts at possible fixes, it has found it again. But so far that has only proven temporary. So far this has happened 9 times with varying periods of time in-between when it would not recognize the card no matter what I tried.
I don't think it's heat warping. I've felt the card, the phone, and tried an IR thermometer and haven't seen unusual heat. I don't think it's dirty contacts. I've tried cleaning with microfiber, isopropyl alcohol, and 2000 grit sandpaper. I don't think it's the tray because the phone will recognize new cards when inserted and I haven't seen that fail no matter how sloppily I loaded the card in the tray as long as it was seated. I don't think it's the way the tray is seated for the same reason and because reseating, no matter what angle or depth or any other variable I could think of, didn't seem to bring a lost card back.
One thing specifically I haven't tried lately is a complete phone wipe and reset. So far I haven't wanted to reload everything again. However I did have Best Buy do a "software flush" and fix then wipe the first time this all started after the first software update I received. The software flush didn't fix the issue but the wipe after that did. That time. But others here have reported doing a wipe and not having that fix the issue so that isn't a provable fix yet either.
I have no empirical proof but it feels like the phone is actually forgetting the card and choosing to ignore it. Almost as if something is scanning and then remembering that specific card and removing it from the system on purpose. But again no proof of this. Still I might just start locking off specific apps just to see if my card comes back.
Until this is fixed it's a real disappointment because otherwise this seems to be a great phone.
And so I left my phone off for about 2 hours while I typed the last update and copied all of my SD card files. Reinserted my main SD card and powered it on... Now the phone sees the SD card. We'll see how long that lasts.
Just popping in to agree with all the rest, in case Samsung IS actually looking into this as previously stated. I have tried all of the options and am having the same random eject issue as the others. It's to the point where I had to move my photos and videos back to the internal storage because I woud take pictures and they would be unable to save because the SD card would randomly unmount and the photos would just get erased. Don't put anything important on your card, or use it for anything that you REALLY need to access, because the likelyhood is that when you need it you wont be able to access it.
I have tried a different card, formatting them with the phone, reseating them (even though they were perfectly seated in the first place), rebooting, and resetting. Nada. Still have the issues.
Been 10 days now of zero problems - since I reseated the card (which took some doing to get it perfectly flush) and cleaned the card's contacts. According to SD Card Monitor not one single unmount.
Music, photos and videos (all on the card) now working perfectly No music or video stuttering, no grey place holders and photos all perfectly ok.
Be interesting to see in which countries owners are having the issues - as different countries / regions get different versions (both hardware and software) of the phone. I wonder if it is mostly US shipped phones? Just a thought. I'm in Western Australia.
I popped into my local Samsung shop and none of the guys there had heard of the issue (or so they told me).
Using a slower samsung evo 64 gb sd card (orange) no unmount/remount based on sd card monitor for 24hrs, and apps are installed in it no gray icons. Unformatted from a windows phone 950xl.
Was using samsung pro select 128gb, and would unmount/remount every 15-30min
Unsupported speed class?
will report back if no change or unmounts/remounts in 48hrs
I also spent too much $ on S8 (SM-G950U) and S8+ only to see pictures/videos disapperar from the Samsungs 32G SD card. i called Samsung 3Xs and no help - got hung up on when i asked for a supervisor - disappointed Samsung! Everytme i take pictures and videos in S8 and S8+, they disappear from the inserted SD card! i already formatted the SD card and also performed a factory reset - no bueno!