Following a cagey opening at Bramall Lane, Shaun Derry's charges began tentatively probing forward.
Early on, Petrasso beat Jay McEveley on the edge of the area before seeing his shot saved by Blades goalkeeper Mark Howard.
Gary Jones, allowed to play in this game after his Yeovil red card was rescinded, also threatened with a powerful run from midfield before forcing another save from Howard.
Just after the half-hour mark, the Magpies took the lead when comeback kid Mike Edwards converted Jones's corner into the far corner with a volley.
United struggled to pull themselves together following their concession, and save for a Jose Baxter effort on the rebound from a free kick, they offered little and were booed off at half time.
The hosts were better after the second half got underway, with ex-Notts winger Jamal Campbell-Ryce and Chris Basham threatening.
In the 70th minute, the visitors had a great chance to score a second goal, but the unmarked Stephen McLaughlin hesitated for too long and was dispossessed.
It was a huge chance to put kill the game off, and Notts would rue their miss as, moments later, when Bob Harris crossed for Jamie Murphy to head in the equaliser.
United were the better side in the latter stages of the game, but Notts were able to hang onto the draw and a decent point against one of the division's heavyweights.