sd unexpectedly removed

[Android Question]

i have samsung galaxy s4 with 16g sd card. i keep getting the sd card un expectedly removed pop up. and its never been removed, it will pop up about 6 times when i first turn phone on till it settles. and happens at random through out day. had no problems when used it in s3. tried all obvious stuff to sort it, removing it and re installing it, cleaning the card and clearing the gallery cache. can any one help thanks