This is a great example of why I don’t trust external Web sites to hold all my data. It can be tempting to keep your tree just on Ancestry or Geni or any of the other sites, just as it’s tempting to have your photographs stored on Flickr or Picasa. But please please please keep your own copies on your own hard drive (and backup them up!).

And no, I’m not saying you shouldn’t have trees, photos, media on external sites, but keeping all of your data hosted somewhere else is asking for trouble.

NEW YORK (CNNMoney) — It’s every Flickr addict’s worst nightmare: One day, the vast photo archive you’ve uploaded and annotated for years suddenly vanishes. It happened this week to Mirco Wilhelm, when a Flickr staff member accidentally deleted his five-year old account, wiping out 4,000 photos.

Wilhelm had e-mailed Flickr customer service about another user’s account which seemed to be packed with stolen photos — a violation of the site’s policies. In trying to delete that errant account, the Flickr employee accidentally nuked Wilhelm’s.

Unfortunately, I have mixed up the accounts and accidentally deleted yours. I am terribly sorry for this grave error,” the Flickr staff member wrote in an e-mail response to Wilhelm’s inquiry about his vanished account. “I can restore your account, although we will not be able to retrieve your photos.”

Read more here.