Phillip Lyle
There is a known issue with a batch of these cards where they can completely die.  This is known defect (affecting a lot of Galaxy S3 and Note users), so if your card dies, either exchange through Amazon or contact SanDisk for a replacement.  Newer cards are supposedly fixed, but you might want to back up your data just in case.  Mine were purchased in November 2012 and both have now failed in separate phones, unreadable by PCs or phones.