NOTE:  please read the last update (scroll to the bottom) - I'm leaving this review as 1 star as it appears to help others who purchased and had a similar experience.I give SanDisk 5 stars for customer service and 1 star for the product (for those manufactured in the past)  For those newly manufactured products that contain the software ROM fix, I'd give the product 5 stars.To all those who purchased this card due to all the great reviews.... and then used it in a Galaxy S4 (GS4) smartphone for storage, the solution to the problem is not one you can solve without returning the card to SanDisk for a replacement under warranty.  Believe me I tried everything that the Google searches returned as solutions... formatting as exFAT, formatting in the GS4 then copying data to the card while in the phone etc.... nothing worked... and so now please stop pulling out your hair and hopefully you will get the same result which I hope to get soon (just RMA'd and will send it back tomorrow for a free replacement under warranty).The problem (As confirmed from the SanDisk customer support rep when I asked specifically asked him):- When used in the GS4, this card will unmount itself inadvertently (may be true only for cards manufactured before the ROM patch described below was implemented in manufacturing).  This can cause the data to become corrupt (luckily was not my experience) and / or can be very frustrating as most of us are saving 13 MP camera pictures to the card (or MP4 movies that stop in the middle of the movie) and it unmounts when you view the picture in the gallery or worse, right when you take the picture (this was the issue I experienced).- SanDisk knows about this issue.  Again, SANDISK KNOWS ABOUT THE ISSUE AND WILL REPLACE YOUR CARD. Call them (info below) and they WILL take care of it under warranty.  They have a good name for a reason and they want to keep it. (sorry SanDisk about the 1 star, but I thought it was the way to get the message out to those searching for a solution)- The customer rep told me that SanDisk tried to work with Samsung to resolve the issue but Samsung refused to acknowledge a problem exists with this card and some of their phones (especially their flagship GS4 phones).- The customer rep told me that to their knowledge the card works fine in regular cameras (not Samsung GS4 phones with cameras, obviously) and even worked fine in some phones from HTC etc. that could accommodate the 64 GB cards. Hence the 5 star ratings from others.- The solution SanDisk came up with was to write a patch into the ROM of the card itself to specifically address the issue on Samsung GS4 phones (and maybe other Samsung phones like the GS3).  I was assured that the new card I would get would work flawlessly. will see in about 3 weeks.- I purchased my card from Amazon June 2013, so the ones currently being manufactured and the one I should get will have the new patch.  However, as SanDisk manufactures in batches (like most of the industry), old batches of "unpatched" cards obviously exist in warehouses.- I asked about a software patch I could download and was told it's not possible... hence the "hard coded patch" in the card's ROMSo... how to take care of the problem....After speaking with the SanDisk US customer support (link and info below), this is what I learned and did.Go to the SanDisk site1) [...]2) call consumer products support (this is the US no.) 1-866-SANDISK (726-3475)3) Tell them that the card is unmounting itself in your GS4 (I assume this is also a problem experienced by GS3 owners too)4) They will then confirm your information, approve your return (10-15 min approval window by mine was basically instantaneously), and then send you emails with a UPS return label and instructions.5) BTW, they also asked if I had a magnifying glass to ID the serial no. on the card, but I didn't and couldn't read it myself (those numbers are super small printed into the plastic... don't believe me?  take a look yourself.)  I told the rep I couldn't read them and he said no problem, they would process without the serial no.  I assume this is because they send you a new replacement card only when they receive your card you send in.  Makes sense.Well, that's it.  Problem (hopefully) solved.  Let's see in 2-3 weeks.  The rep was super nice about it and seemed like they were all too familiar with the problem.  KUDOS TO SANDISK FOR TAKING CARE OF IT... however, it would be nice if there was some type of communication / notice of the issue besides Google info sources, forums etc.  I spent hours, maybe a couple of days total hours worth, thinking it was an issue with my phone / my data transfer.  oh well, that's how it goes.I really hope that this review helps some of you out there with the same problem as me.  GOOD LUCK!!(btw, I'd give the card 5 stars if this problem was solved in the patched ROM of newer cards as the data transfer speeds are awesome on the card (tested transferring data via a card reader hooked to my computer)... if the data transfer / remaining mounted in the GS4 issue never existed (pre-patch of ROM, of course))UPDATE: 08/17/2013 - All went well with the return of the SanDisk Storage card.  UPS shipping took about 2 days from when I sent it, SanDisk verified the warranty claim 1 day later and 5 hours later that same day, my replacement card had been shipped. waiting on the new card now, but expect to test it out very soon (early next week).  will update the post after testing out the new card.  Just as a reference, I've been using a 32GB Micro SD HC SanDisk card in the GS4 since having shipped the 64GB card.  Obviously it's a different card and not the Micro SD XC kind. Other than a bit slower access speeds to the card, absolutely zero problems.... well if you don't count that it's basically filled up, hence why the 64GB option is great on the GS4. So it proves the point on the problem with the early build Micro SD XC 64GB cards.I'm happy to see that my review has helped some others out there.  Good luck to you in your warranty claim. my experience with SanDisk customer support has been excellent so far.Update: 08/21/2013 - Received the new replacement SanDisk Storage card just a few days ago (shipping back to me once they agreed it was defective was a very short time window).  Have had it running in the Galaxy S4 for a few days now without any jumps, stutters or card unloading in the middle of picture or video taking.  Card seems to work flawlessly as the SanDisk Rep mentioned it would. One would think it's such a short time to test after a few days of use, but if this were not fixed already it would have dismounted in the first 45 minutes... that's how often and how fast it was occurring. so it would seem the problem is solved. The new SD card makes the phone even more responsive when moving between pictures in the gallery and performing other tasks like copying files to the card.  Thanks SanDisk!  Unless I update this review by next week, you will know that the problem has been fixed.Good luck to those with similar problems as we now know how to get it fixed thanks to SanDisks great responsive customer service.Update: 08/31/2013 - This is the last update to this review just to "close the loop".  The new replacement card from SanDisk has been running flawlessly for about 2 weeks now.  I have not experienced any of the initial problems.  SanDisk really did fix the issues in the new batch of cards.Update: 12/09/2013 - The card has been running in my GS4 flawlessly since replacing it around Aug 21st.  Pretty good track record so far (~ 3.5 months) so it appears to be fixed with the newly manufactured (replacement) cards.For those that have left comments that I should change the rating upwards now that the issue is fixed (see my addition at the beginning of this review)... When searching for a solution to a problem with a purchased product, most people start at the 1 star reviews to see what others have experienced.  This is especially true for products with a lot of reviews (this is what I did in the beginning with this product).  Therefore I think leaving it at 1 star and adding the comments at the beginning should clarify the review.Thanks to review commenter "Da Shocker" for his input.  I think he sums it up very well when it comes to the value of files stored on media like this:"One more thing.. to those not happy with the one star rating by this person go ahead buy one. Take tons of pics vids and save some music on there then when the card crashes come back and give it a five star. A card cost $50-$100 pictures from a vacation or family event are priceless.. "