Nokia Lumia 521 - Why can't my Lumia 520, 521, 525, or 526 phone find my memory card?

1). Check that your memory card is fully inserted into the memory card slot.

2). Check that your memory card is formatted in file system FAT32 or exFAT

3). Try to insert your memory card into Card Reader and format it. If you have important information on your memory card, copy it before formatting.