Having looking at the photos on the SD card from my DLSR camera with a USB-mounted card reader plugged into my new Tab S8, I found that unmounting the USB required me to restart the tablet after a safety warning.
I could not trace any appropriate guidance in the user manual. I found the answer after some digging around:
Open the 'My Files' app, look on the left for confirmation that the external USB SD card is shown, tap on that section, then go to the three dots at the top right. This gives the unmount option.
I hope this will be covered in the next update of the user manual.