I have two 3Tb drives (LaCIE). On one of them are all my files - I do not keep files on the hard drive of the computer, I work from the external drive. In this way, if my computer has any major breakdown, I can plug the hard drive to the laptop and keep working without interrumptions (this once saved my life on a deadline!). The other drive is a time-machine backup of both my computer AND the documents drive. Additionally, I occasionally transfer key folders to Dropbox (I have a paid account, so a lot of space) for extra safety. I also use a WLAN connected drive, but only for transfers (too slow for working or for backup).
It seems paranoid, but I work with computers of all type since 32 years and I have seen ANYTHING happen (including two hard drive breakdowns, with total loss of all data).