Seems like even a saddle bag would have the same vibrations as a pannier. Only the backpack/messenger bag would have you as a shock absorber to cut the vibrations. Going to a solid state memory would solve that problem. As always, I think whatever you decide a regular backup protocol is key.
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Bikes: 1996 Eddy Merckx Titanium EX, 1989/90 Colnago Super(issimo?) Piu(?), 1990 Concorde Aquila(hit by car while riding), others in build queue "when I get the time"