I have a similar problem today. Only had my S4 for about 3 weeks. Woke up and spotted the SD card has been removed message in the notification bar. I coulnd't remount the card from the settings so I rebooted the phone but still it shows as Mount SD card / insert SD card in the settings under stoarage. I turned the phone off removed the card, put it back in and restarted the phone but still the same issue. I have tried 2 other SD cards and both get the same problem so it's not card problem.