So today my sd card had an issue today where i couldn't play videos through the stock Samsung video player. I tried to play 3 vids and was persistend with one of them. I rebooted the phone to see if it would work and the 3 files gave me the sorry couldn't play video , 2 of them with thunder bolt thumnails. So I downloaded MX Player and this is the current state of things. Ps. Recently my camera app started to continue as if I was filming when I pressed the stop button but little later the recording would stop and the video would not appear to have added the additional few seconds of recording.
File 1:
Can't be opened by Video and has no Gallery thumbnail but has one for MX. This file can be opened by MX Player although it has 2 instances of part discoloration that last 1-2 seconds. If opened by Windows media player ot tesults in a format/codec error. But details are accurate.
File 2:
Can't be opened by Video and has a Gallery thumbnail. There is a split second of sound before the error. This file can be opened with MX Player although firstly it plays the vid until 4 seconds followed by a quick grey screen and then the video starts from the beginning and plays the whole video. This has a Gallery and MX Player thumbnail and accurate details. Same windows error as above.
Now the most problematic file,
File 3:
No thumbnails, inaccurate details (says 0:00 seconds playtime and has multiple creation dates), can't be played by Video nor MX Player, same windows error as above. This is the file I tried to open several times before rebooting.
All the files have correct file size as before the problem.
Phone: Samsung galaxy s4 (GT-I9505) running Android Kitkat 4.4.2
File 1:
Can't be opened by Video and has no Gallery thumbnail but has one for MX. This file can be opened by MX Player although it has 2 instances of part discoloration that last 1-2 seconds. If opened by Windows media player ot tesults in a format/codec error. But details are accurate.
File 2:
Can't be opened by Video and has a Gallery thumbnail. There is a split second of sound before the error. This file can be opened with MX Player although firstly it plays the vid until 4 seconds followed by a quick grey screen and then the video starts from the beginning and plays the whole video. This has a Gallery and MX Player thumbnail and accurate details. Same windows error as above.
Now the most problematic file,
File 3:
No thumbnails, inaccurate details (says 0:00 seconds playtime and has multiple creation dates), can't be played by Video nor MX Player, same windows error as above. This is the file I tried to open several times before rebooting.
All the files have correct file size as before the problem.
Phone: Samsung galaxy s4 (GT-I9505) running Android Kitkat 4.4.2
It's unfortunately a relatively common issue here. Try unmounting the card (Settings>Storage), removing it, and then inserting it into your computer using a fullsize SD adapter. Can you play the videos there? If so, back them up now. Then run chkdsk (on a Windows machine) on the SD card to see if there are any bad sectors: 
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