In theory, cloud storage is great. Instead of some $150 hard drive connected to your PC, your data rests on industrial-strength corporate servers, backed up, powered by redundant systems and attached to uninterruptible power supplies. What setup could possibly be safer?

How quickly we forget…

  • MobileMe Becomes a MobileMess: Apple’s own MobileMe service (now called iCloud) offered automatic syncing of e-mail, calendar, address books and other data across various Apple gadgets and Macs. But its history was rocky. It erased user data or faced unexpected shutdowns many times during its three-year life: summer 2008 (11 days, resulting in permanent data loss in some cases), December 2009, September 2011 and June 2012, for example.
  • Gmail Goes Dark: Millions of people trust their e-mail lives to Google’s free, excellent Gmail service. You access it on the Web, so it’s available from any computer. Which is all great—until it goes down.

In February 2009 that’s what happened. When you tried to log in to check your mail, all you got was a “502 server error.” That outage lasted only a few hours, but it wasn’t the only one. Gmail had blinked out before, and would blink out again—for five days in 2011, for example.

PayPal, Intuit (maker of Quicken), Rackspace, Terremark, Salesforce and many other companies have also landed in the headlines when their online services went down.

The point isn’t that cloud services are unreliable; on a percentage basis, they’re astonishingly reliable; most run smoothly 99.9 percent of the time, for example. They are not, however, infallible—and they should also be optional.