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The main difference is that the use of porous disk implies that the solutions are not in separate beakers and allows electron flow; overall the function of the porous bridge is same as that of the salt bridge: prevent charge buildup on one electrode.
A quick correction to some of the answers: a salt bridge or porous disc allows ion transfer, not electron transfer. Electron transfer takes place along the wire (hence the electric current). In the Cu/Zn galvanic cell example, the salt bridge provides anions to the anode beaker because Zn is being oxidized (the concentration of Zn2+ ions in the beaker increases and the anions balance out the charge). Conversely, the salt bridge provides cations to the cathode beaker because Cu is being reduced (the concentration of Cu2+ ions in the beaker increases and the cations balance out the charge). If there were a porous disk instead of a salt bridge, the Zn2+ ions would move towards the cathode half of the cell and balance the charge in both halves of the cell.
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