In some cases the SD card may becomes unreadable or unfindable by your Remix devices. As you find that the card is still usable, this issue may be resolved by connecting the card to a computer (Windows operating system) using an external USB card reader.
If the card is recognized when connected to the computer using an external card reader you can follow the steps below to recover your data and restore your card to working order in your Remix devices.
1. Back up all data on the card.
2. Format the card using the computer.
3. Connect the card back to your Remix device.
HINT: You can also try ES File Manager as an alternative.