Its seems to be all makes of cards. I have a Samsung 8GB card that my S4 has decided to unmount and will not remount, no matter how many times i try a soft reset. It will not identify that the card is even in the phone to format again.
My galaxy s4 (19505 ) has just started the problem of not mounting the SD card. It is an 8GB Samsung SD.
No matter how many times i do a soft reset , it still says the card is unmounted.
Clearly it is not just a problem with my phone but seems fairly common