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I have a Samsung tab Pro 10.1 and when I insert a new micro sd card of 16GB or of 8GB the card won’t mount. The option at ‘settings>general>storage>mount sd card’ is greyed-out and therefore not possible to mount the sd card. Can anyone help me solve this problem?

Make sure you are formatting them to FAT32, not exFAT. I believe Android only reads FAT or FAT32 on older android builds.