I wasn't comfortable using Dropbox at first either. I started with small things to test it out before working my way up to larger files and things I did not want to lose. I do store my fanfiction writings on Dropbox but if I was doing original writing I'd review the service contract to make sure they can't claim my work because they are hosting it. It may not hurt to ask around and see what other writers use for cloud backup.

Normally drives should last at least 5 years, it sounds like you've been exceptionally unlucky with hard drives lately.