I am sorry if I sound repetitive(I've asked a similar sounding question earlier - Relevance of memory reads while calculating the time complexity of an algorithm). Actually, it didn't answered my intended query. May be I framed the question inappropriately, and in an unnecessarily complex and indirect way.
In the current/present question, I am referring to the magnetic disk memory as it definitely takes much time, but if the primary memory(RAM) also does take much time relative to processing, then it can too serve the purpose here.
Also, I am specially concerned here with the algorithms where number of memory reads are some function of 'n', instead of being nearly constant(because in latter case, this question is not applicable or doesn't make sense anyway).