synctoy Microsoft SyncToy is the best utility you’ve never heard of.

SyncToy automates the process of copying a folder and all of its subfolders to an external hard drive or another computer on your network. Once it’s set up, it efficiently keeps the two copies in sync – adding new files, replacing files that have changed, deleting files from the copy that were erased from the source.

That’s not all it does but that’s the important thing for most people. It’s free, it’s easy to set up, and that turns out to be a pretty wonderful thing to do.

You probably have a backup program. (You do have a backup program, right? Windows Live OneCare is doing backups, or you’re using Vista’s built-in backup features, or you have some other plan, right? Make it be true!) Those are wonderful things and you must keep doing them. SyncToy should not be your primary backup plan but it can provide an additional layer of protection.

Sometimes it helps to have an extra copy of your files – no muss, no need to run a program to restore anything, just another copy of your files. SyncToy makes that happen. In SyncToy, you browse to the source folder – My Documents, or My Pictures, say – and then browse to an empty folder on the external hard drive, or on another computer. SyncToy copies all the files from the source to the destination. The next time you run SyncToy, it makes whatever changes are necessary to sync the two folders in one click. And just that easily, you have another copy of all your family photos, or your music library, or your work documents, or your manuscript, or whatever deserves that extra bit of attention.

There are several different options for setting up that sync operation. Personally, I focus on “Echo,” which is a one-way sync – all the changes in the source are mirrored in the destination when I run SyncToy. If I’m worried about accidentally deleting files and not noticing for a while, I set up SyncToy to “Contribute”, where changes to files are synced but no files are ever deleted from the destination.

SyncToy effectively has no limits on number of files or file sizes. I use SyncToy to make a copy of a library of than 40,000 files in thousands of folders totaling 325Gb. It takes about ten minutes to run SyncToy on that library and bring the copy up to date. In addition to your documents, photos, music, and movies, you might want to use SyncToy to make an extra copy of Quicken and Quickbooks data, or data from other programs; Outlook .PST files; Internet favorites; or saved games.

The important thing to remember is that SyncToy does not run continuously or automatically. It requires manually starting it and running it. I set a reminder for myself to run it once a week. Theoretically it can be scheduled to run regularly but I’ve never had much luck with that.

The new online services for syncing folders among multiple computers can give you similar peace of mind in a similar way. You can choose which method works for you; you probably don’t need to use all of these simultaneously. Windows Live Sync can be set up to sync folders on computers that are not on the same network. Live Mesh does the same thing and adds a copy stored online as well.

SyncToy has the virtue of being easy and fast and it makes me happy when I run it. If you have a big external hard drive and you want the peaceful feeling that comes from knowing you have an extra copy of the family photos, give it a try!