This might be a bit late but I'm just posting so that others having the same problem can benefit from this.
After the described problem above happened, I gave up on retrieving the files on the card after many many failed attempts in various ways (copying individual files, backing up the card, used some 3rd party apps) but nothing seemed to work.
So I went ahead and formatted the card to FAT32 using my laptop, copied a few songs to it and it seemed to work well. Few hours later, the same problem happens. I tried using the exFAT format, did the same with different songs (to rule out the songs themselves being corrupted), but still had the same problem few hours later. My last trial was formatting it to FAT32 but using my galaxy S4 this time, and with no surprise, the same problem persisted. At that stage I gave up on the card.
I contacted SanDisk support using the live chat feature on their website, before I told him my problem, I asked him to check if my card has the new firmware or not (they manufactured some faulty cards and blamed it on the old firmware they used). He told me it did have the new firmware, and then I realized that my card was corrupted beyond repair. He asked me for some information to ask for a replacement. I didn't replace it because shipping it back would cost a lot, but id you're living in the states then just replace it.
On another note, I am not going to buy another SanDisk card, I will be trying samsung cards as they seem to have very good reviews.
Sent from my GT-I9500 using XDA Free mobile app