-technology evolves rapidly, there is no reason to fear for the safety and reliability of an external hard drive not to mention the advantage of having a much lighter computer, because you do not have the load of data which reduces both reliability and speed;of course protective systems against viruses and so on acts also on the external drive, every time you connect it to your computer.
-then you can always use more disk partitions.Or use more than one internal hard disk,whether the structure of your machine allows this.
...however, one can often recover data even from a hard drive crash.
Sorry,I know, this is the Paulina's thread,just I try to do some advice, for sure your Mac-person know more than me!