How to backup on a external hard drive that is not always connected?

I didn't use this much on Win7 (just once, it works) but used this more often on WinXP laptop & this worked perfectly for me for usb keys/drives:

http://mattcollinge.wordpress.com/software/usb-detect-and-launch/

Does what it says on the tin, free for home use & you can use the setup as described in scripting examples.

You set it to check every X seconds, then it sleeps until execution time, and will not run again unless the app is restarted or the usb drive is reconnected.

I have not used this software myself and i could not find any free software that meet your requirements. Hope it's what you are after. This program offers a trial period. http://backupusb.com/
Adware has been reported with trial.

I use Crashplan. It has a free option, which you can use to backup to an internal/external drive, or to another computer, either in your local network or not. When the backup destination is not available, the backup will not be executed. To be clear, I don't see how this would be different from the Windows Backup.

I have the family pack, pay $12 per month and can backup any computer that I control with unlimited storage. The only limit is upload speed. Then I have a local server at my home, which I use as second backup. I backup several family computers to this computer. Another plus: it works on Windows, Linux and OSX. The only catch: there is only one key to encrypts backups, so whoever has that plus the CP login, can open all backups.

You can backup to a friends computer for free, if he/she has CP as well. They can generated a code, give that to you, and vice versa. If you encrypt the backup, you cannot see the contents of the backup.

http://www.crashplan.com/