If you face any issue regarding the mounting or unmounting of SD card on your Galaxy S4 then no need to think much for that as it usually happens even though you do the insertion and removal of your SD card in a proper way. There are some reasons behind this issue, and these are:
> Either the phone or SD-card is not formatted
> The SD-card slot is damaged
> It happens sometimes due to a temporary phone problem.
> Your phone does not support the SD-card
> Any application is running in your phone, which is not allowing unmounting the SD-card.
Here are explained the possible solutions to fix SD-card unmounted and damaged error on Galaxy S4.
Methods to fix SD card unmounted and damaged error n Galaxy S4
Considering the problem is related to a temporary phone problem go for rebooting or reset your phone once after taking the battery out for few seconds. The problem will be fixed if this was happening due to the phone problem.
Find out the specification of your SD-card and which the Samsung Galaxy S4 supports. Compare them and if you find them different then you need to exchange the SD-card.
Check the SD card’s connector, and it should not be damaged, missing or pushed in.
If the SD-card is not correctly formatted, or it is corrupted, then insert it in the computer and see if your computer can read the SD-card. Take all the back up and format it again to work properly.
Sometimes this problem occurs due to the corrupted file system on SD-card, and you get the SD-card unmounted and damaged error, which can be fixed using Desktop windows’s built in chkdsk utility.
Start the command prompt (start->run->cmd.exe) and then type chkdsk/X/F > SD card drive letter > and you will be able to fix SD-card unmounted and damaged error on Galaxy S4.