You made me curious enough to do the math. Indeed, two seeks per line (one to read, one to write) at 10 ms each add up to 56 hours(!) if you assume a set of 10,000,000 lines to randomize. In a 4GB file with 10,000,000 lines a line would be 430 bytes long on average, so it's probably safe to assume that the OP's actual problem would require 4-8× as much time. Waiting a week for the randomization of a 4GB file doesn't sound very appealing…

Makeshifts last the longest.