Unmounting an SD card means disconnecting it on the software side without physically removing it from the slot. It is a good idea to unmount an SD card before removing it to avoid problems like corrupted data. Unmounting an SD card makes it safe for physical removal, and once you unmount or eject the SD card, the device will no longer detect or use it. Unmounting an SD card is important because not performing it before the physical removal of the SD card can cause irreversible damage to it, including data corruption.

To unmount an SD card on an Android device, you can go to Settings > Storage > SD Card > Unmount/Eject. Alternatively, you can tap all the way through to your SD card, then tap "Eject". Some older phones will have the "Unmount" option instead, which does the same thing. It is important to make sure you are not writing or reading any files from the SD card before ejecting it.