Xiaomi
Mount SD Card auf Xiaomi
MIUI treats an SD card as removable media, similar to a USB drive on a computer. Once you eject it manually or it gets unmounted after a system prompt, Android stops writing to it and hides it from the storage overview until you mount it again.
This protects your files from corruption if you pull the card out physically while it is still active. The downside is that a simple reboot does not always bring the card back automatically, especially on older MIUI builds.
To remount the card, go to Settings ➔ About phone ➔ Storage ➔ Mount SD card ➔ Mount.
If the option is greyed out or the card still does not appear, the card itself may be unreadable or damaged. In that case, remounting will not help and you need to check the card on a computer instead.
Remounting only reconnects an already recognized card. It will not fix a card that shows up as blank, asks to be formatted, or was never detected by the phone in the first place.