::::The data coming off the RAMAC read/write head is an analog stream requiring a very substantial amount of processing by 1956 standards to recover digital data; u might want to read "A Self-Clocking System for Information Transfer" L.D. Seader, IBM JRD, April 1957, which has sketches of the analog and digital signals.  To my eye they don't look a lot different than today's signals - just way lower frequency.  The signal out of the BlueRay channel is at one point a digital bit stream that would be trivial to interface to any computer.  There is no difference other than time differences in technology and that in the BluRay case the digital bit stream is not exposed - it is there and is just a pair of wires! 
 ::::I'm pretty sure Shannon would not understand 0.61 bit.  I do agree with you that at the drive interface it is not possible to distinguish one bit from another, but that is only true in the absence of format information.  IBM tells us their format includes 6 data bits and elsewhere they tell us the 6 data bits map to 48 states leaving 16 undefined states.  The fact that states are undefined does not mean they are not available and therefore the information content remains 6 bits.  There is probably some information theory law of conservation that states something like the information content of a state space is that minimum number of bits whose number of states exceed the number of valid states in the state space.  That's how channel encoding works, regardless of whether it is a disk recording channel or a communications channel.