I have an iMac with a 500 Gb disc. I have owned the machine for a few months now and am considering getting a portable external drive for backups. So far I have only used about 50 Gb of the disc after transferring most of my stuff from my old PC and cannot see myself using much more than that for a long time. Most of the advice I have seen says you should get a backup drive at least twice the size of the disc to be backed up. My question is do I really need a 1 Tb backup drive bearing in mind the fairly small amount of data to be backed up? Would 500 Gb portable drive not be enough? Thanks.