We've all been there: that place where you've just completed a project and are ready to sit back and admire all of your hard work. Then, without warning of any kind, you lose whatever you've been working on because you haven't saved it. Or, your computer crashes (who hasn't had this experience?) and leaves you with nothing. In that case, saving to a drive that no longer works or is inaccessible, won't save you or your data.
The solution is a backup separate of your computer, in conjunction with a power backup. I also save certain, larger files, to disk, on top of exporting them to my data protection service of choice: Carbonite.com
It's not a life-altering experience to have to recreate a sentence-or-two, but a page, a document, etc. can set you back and disrupt your ability to stay focused on your writing. You can now purchase portable hard-drives to create on-site backups, but I do recommend something like Carbonite to supplement that.
You can also adjust your word processor settings to save more often. The default setting for most word processors is to save at ten or fifteen minute intervals. Depending on fast you write, ten or fifteen minutes can involve a lot of lost prose. I've adjusted my settings to save every minute. Sometimes this slows things down, but I know that I'm not going to lose bits-and-pieces of my writing for want of a saving my work.