It seems the ultra series of cards have a very high failure rate. When they fail, they go into read only mode so that the existing data can be retrieved when it fails. I would not suggest storing sensitive data on these cards.
Fortunately they come with a lifetime warranty from San disk. I just rma’d a 64GB ultra and they sent me an Extreme series. Hopefully more reliable.
One thing that irks me about this industry is that they get away with false advertising so openly. The 64GB card only has 59GB capacity. Without explanation I might be inclined to say it is a reserve for bits that go bad in the future but no. They have arbitrarily redefined 1GB to 10^9 instead of 2^30. I hope they next time the ceo of SanDisk goes to the grocery store and buys a dozen eggs the container will only have ten and the one pound steak will only be 10oz because the supermarket redefined what a dozen and a pound is.
