Aged 33, the former Doncaster Rovers and Wolverhampton Wanderers star spent a successful two-month loan spell from the Deepdale Ground club last season at Bramall Lane.
Warnock said: "We're delighted, obviously. Mark represents something slightly different because he knows what it's all about here."
Former Leeds United, Manchester City and Bolton Wanderers centre-back Mike Whitlow has joined the Blades on trial and will sign for the Blades should he impress the Blades boss in pre-season friendlies.
Meanwhile it has emerged that Shaun Murphy has been released from his contract. Blades-Mad understand that Neil Warnock wishes to reduce his squad in order to bring in new players but we believe that the former West Bromwich Albion defender wished to retire after his wife discovered she was has cancer.