Is it a new card? If so, its already formatted FAT32, and should work out of the box. If its still asking you to format, even after formatting on PC, and format on the phone doesn't work, the card may be defective or damaged.
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Phone: HTC One X (AT&T version) Rooted, Bootloader Unlocked, S-Off
Tablet: Samsung Galaxy Note 8 (WiFi only) Rooted