Mark penned a new contract with the Blades in the summer of 2014, following an impressive second-half to the 2013/14 campaign after dislodging George Long in early February.
The experienced goalkeeper proved to be a reliable last line of defence as United kept nine clean sheets in ten-match winning run in all competitions and the stopper was also instrumental in helping the Blades reach the FA Cup semi-final.
Last season Mark started 50 games, despite a troublesome back injury.
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