Oppo
Format SD Card auf Oppo
Oppo phones ask you to format an SD card whenever the file system on the card does not match what Android expects, or when a card was previously used in a camera, laptop or another phone. ColorOS treats an unformatted or incompatible card as unusable, so photos, music or app data cannot be stored on it until the format process runs.
Formatting also removes every file from the card at once. This makes it the quickest option when you want to hand the memory card to someone else, reuse it in a different device, or simply clear space without deleting files one by one. There is no way to undo the process afterward, so move anything you still need onto internal storage or a computer beforehand.
Menu path on Oppo (ColorOS)
Open Settings ➔ Storage ➔ SD card ➔ Format SD card. Confirm by tapping Format SD card again on the warning screen.
If the card still is not recognized afterward, try it in another device or check whether it is physically damaged. A card that fails to format anywhere is usually defective rather than a phone software issue.