Well, to be technical, "writing onto" the disk will change the physical state of the disk in some way, presumably by putting the atoms in different physical states. If these different states have different energy levels, then storing information on the disk will impart (or take away) some amount of energy which, according to E-mc2, will change the mass of the disk.
However, this change in mass will be negligible (literally -- I doubt it would be measurable even with the most sensitive measuring instruments). Besides, if one is having a theoretical discussion then one can posit a storage medium where the "on" and "off" state of each bit-storing element has exactly the same energy.
This world can take my money and time/ But it sure can't take my soul. -- Joe Ely
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