Based on the number of posts in this official forum, there is clearly a defect with at least this particular microSD card sold by Sandisk.
I purchased my card in December 2012 from a brick-and-mortar RadioShack, hoping this would minimize the chances of getting a counterfeit card. I did not actually begin using the card until late February 2013. The card is now completely dead, unrecognized by my Galaxy S3 or my Windows 8 laptop. Actually, the laptop does recognize it about 1 out of every 5 attempts (using a 5-year old Sandisk USB adapter), but says that it is write protected. During its death, the card exhibited ALL of the symptoms mentioned ad nauseum in other threads on this forum (uncommanded dismounting, card not recognized by device, etc.)
There have been many posts regarding these cards failing in smartphones, particularly Samsung phones. However, I think it is neither a phone nor an OS issue. I also have a 5 year old Sandisk Ultra Mobile 4GB Class 6 microSDHC card that works flawlessly. This card worked without issue on the Nokia E71 for which was originally purchased, a Motorola Atrix 4G, and the Galaxy S3 for 4 months prior to my switch to the 32GB card. I have now reverted to this lower capacity card while I attempt to RMA the 32GB card and see if SanDisk has decided to fix the problem with the current Ultra series cards.
If the problem is of a technical nature, i.e. the phones can’t handle the random access time properly as was mentioned in another thread, the SanDisk needs to advise users of this incompatibility and ensure consumers make an informed decision with regard to which microSD card with work for their intended application. Currently, the SanDIsk website clearly indicates that the Ultra series of microSD cards is “best for smartphones”. Apparently, this may not be case if smartphones like the GS3 can’t handle the “high read/write, low random access” speeds of a Class 10/UHS-1 card.
If it is a manufacturing defect, as mentioned in the UK Register article, then SanDisk needs to own up to its mistake and begin offering replacements rather than making consumers find out the hard way. I will attempt only RMA and if the problem is not fixed. I will never purchase a SanDisk product ever again as it is obvious that product and manufacturing standards have slipped beyond any minimum acceptable level.
This defect is significant enough to warrant yet another new post in this “official” manufacturer forum.