I did get the name of the ship wrong, but it was Lincoln who secretly ordered the ship removed from the expedition. Without Lincoln's signature Porter could not have taken the ship away from Mercer. Lincoln indorsed the plans of Seward, which included bypassing the eyes and ears of the Secretary of the Navy, Welles, and the Sumter fleet's commander Fox. When the plan was found out, Welles was furious and asked the president for an explanation, and he refused to disclose the secret plan. Lincoln then had Seward restore the Powhatan to Fox's fleet. Seward did not rush to send the orders and the late telegram was not signed by the Lincoln but by Seward, a fact everyone knew was not binding to the officers in the field, acting on the original direct order of Lincoln. Mercer, trying to keep to his schedule with Fox's fleet, showed the order to Porter, who saw Sewards signature and ignored it and kept to his schedule of the Fort Pickens expedition and left with the Powhatan to Pensacola; Fox's fleet meanwhile, leaving for Charleston, never knew the most important ship in the operation was now on its way to Florida, dooming the Fort Sumter reenforcement operation.
So why was Fox's fleet left without its major player and allowed to show itself outside the Charleston Bar? To entice the Confederates to open fire on the fort first, distract the Davis government from the real objective of reenforcing Fort Pickens, and win the moral and political high-ground. Why was this kept secret? Lincoln knew the officers in Fox's fleet could not be trusted in such a plan, one captain, figured it out at the last minute and refused to have his ship approach the bar when ordered by Fox, stating he would not take part in starting a war.