I'm a little confused by the two terms. What do data rate and data volume mean and what are the differences?
Data Rate means the amount of Data per time unit. For example 100 Mega Bytes of data per 1 second of time or 800mbps (Mega Bit per second assuming each byte is 8 bits).
Data Volume means total amount of dada. for example 1 DVD can accomodate total of 4 or 8 Giga Byte data.