Hi, and welcome to the UK community
Normally one would swipe down into the notifications and there should be an option there to unmount the device
Alternatively, look at the below link for tips
https://www.samsungsfour.com/tutorials/how-to-safely-unmount-a-usb-storage-from-a-galaxy-smartphone....Also, after any update, it's advisable to wipe the cache partition, which may resolve issues like this.
Plug into a working PC or laptop or other Samsung device
https://www.samsung.com/uk/support/mobile-devices/how-do-i-clear-the-cache-on-my-device/Or follow these instructions
1. Turn off the device.
2.Connect To Computer , other Device or TV USB. Press and hold the Volume Up key and then press and hold the Power key
3. When the Android logo displays, release both keys.
4. Wait until the Android system recovery menu options appear.
5. Press the Volume down key several times to highlight wipe cache partition.
6. Press Power key to select.
7. Press the Volume down key to highlight yes, them and press the Power key to select.
8. When the wipe cache partition is complete, Reboot system now is highlighted.
9. Press the Power key to restart the device.
