Not saying I've figured it all out, but I didn't have a random restart, freeze-up, or unmount in over a week with a 128gb SD card in my S8. And now I'm 3 days into a 256gb SD card --- still going strong (knock on wood). I won't list everything I did to get to this point (which is a lot), but here's what I did to arrive at my current level of success. Give it a try and let me know if it works for you... Format the SD card on your PC. Don't do the Quick Format. Power down your phone and insert the newly formatted SD card in the phone. Make sure it's inserted into the holder REALLY good. Should snap into place and not fall out when you wave the holder around a bit. Power up the phone and go to Settings -> Storage and then format the SD card in the phone. Unmount and Mount the card several times. If it doesn't do this cleanly, power down, remove the card, and re-insert it into the holder. Restart the phone and go through the process of formatting the card in the phone again and unmounting / mounting until it does this at least 5 times with no issues. Keep doing this last step until you're golden. At this point, you should be able to put stuff on the external storage. Take it slow and don't FULLY trust it with your important stuff until you've had a few days with NO issues. That's it! So far, it's worked for me. After I did this, my phone was never powered down in over a week. I have music, video, pics, and apps loaded on the card (some that I use a lot as well as ones I rarely use at all). The SD card still unmounts and mounts cleanly (takes longer with more stuff on it, so be patient and wait for it or you might get an unwanted restart and a "corrupt sd card" message (try to restart before believing you have a bad card). Hope this helps!
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I truly hope you have some influence with the Samsung problem solvers to get their attention about this. I am having these same problems, too. Before I was aware of how widespread this problem is, I spent many days and hours trying to resolve it myself. I can tell you, I do not believe this is an SD card issue. If it is a hardware issue, it surely is with the phone itself. Hopefully it ends up being a solveable software issue which can be corrected with an OTA update. Here's an abreviated look at my problem and troubleshooting history: Got my new S8 phone and Samsung EVO Pro Select 128gb SD card. Set up the new phone and Smart Switched all my stuff from my S7. Phone kept randomly locking up and restarting. Other times, the SD card would just unmount or not show up during the restart. At this point, I thought maybe it had something to do with compatibility issues from older apps or software from the S7, so I did a factory reset and complete format of the SD card (both a complete format and then again while mounted in the phone). I next did a complete "naked" install from trusted original sources (Google Play Store, Samsung Apps Store). Still had the same problems. Now I'm thinking "bad phone". Called Verizon and got a replacement exchange. Also, returned the SD card to Amazon for replacement. 2 days later, I try it again "naked" with the new phone and card...same results. Now I'm thinking "SD" card, so I try 3 different brand name cards in 3 different capacities that were known to be good. Again...same results. Next, I removed the SD card and did another factory reset. This time, I only loaded apps on internal storage (never put the SD card in the phone). NO ISSUES WHATSOEVER! 2 days later and still no issues, no spontaneous restarts. Next, I put a 256gb SD card in the phone, formatted it, and didn't load angthing on it to see what would happen. Guess what? Problem returns. There are a lot of people out there that have a lot of time and money invested in these flagship phones. Many are probably unaware of all that's going on and are frustrated like I am. This needs to be fixed! Hope this helps. - One Aggravated S8 Owner
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