Smartphones are more like computers these days and a simple reboot could sometimes fix the most complex errors of them all. You can still restart your device, but shutting it down and switching it back on again has its own advantages. When you undergo this process, the cache memory will be completely cleared and the Android OS will be freshly loaded into it, allowing it to possibly detect your memory card.
Press and hold the power button
A couple of seconds later, a pop-up screen will emerge
Tap the power off and let your phone shut down completely
Wait for at least 30 seconds and press and hold the power button again
The Android OS will boot up
This is seriously SO frustrating! When the error pops up my pictures in the gallery (the pics that are saved to SD) will turn black or so blurry you can't show them to anyone. I just got this phone a month ago and it started happening a week in. I just did the latest software update and 5 min after the restart the error popped up again. Come on Samsung, you're better than this!!!!
I just got my phone yesterday so am not familiar with everything yet. How do you clear the system cache? I'm having the same problem on my phone. I transferred the SD card from my S9 to this one when I purchased it. Thanks.