How to import, export and share photos with Photos for Mac .

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Some photo programs will find all the pictures on your hard drive, and the leading free candidates are Microsoft's Windows Live Photo Gallery and Google's Picasa. The latest much-improved version of Windows Live Photo Gallery 2011 might be my first choice but it needs Vista or Windows 7, so you'd have to go for Windows Live Photo Gallery for Windows XP.

Enter a Windows Live ID (such as a Hotmail address) when it starts because this links to 25GB of free online storage space in SkyDrive. WLPG will automatically sort all your photos by year, month and date (without moving the originals), but running either WLPG or Picasa should find any photos you have missed. They frequently find something embarrassing.
As mentioned, many Windows programs will find identical files even if they have different names and dates. I've used DoubleKiller for some years because it's small (392K), reasonably fast, and runs from anywhere. (You don't need to install it, so you can keep a copy on your handy utility USB key.)

Good alternatives include the Auslogics Duplicate File Finder, which is free for personal use, and Duplicate Cleaner, which might include a Bing toolbar (you can decline it).

There are also programs that have special features for finding duplicate photos, such as VisiPics and Anti-Twin. VisiPics will find pictures that are not quite identical because they are in different formats (jpg, gif, bmp, png etc), different resolutions, or have been edited. It shows thumbnails side by side, and moving the mouse from one to the other shows the changes.
When you have your digital photos consolidated and de-duped, you can copy the whole directory structure to your external hard drive. After that, you can set up a simple program to keep the two directories in sync. Microsoft's free SyncToy 2.1 will do the job, as will Jumping Bytes' PureSync. I tend to do it manually using the open source FreeFileSync.

Remember that having all your photos on an external hard drive does not constitute a backup unless they are still on your PC, or on a CD, or on SkyDrive or whatever. Schofield's Second Law of Computing says data doesn't really exist unless you have two copies of it. Preferably more.