My girlfriend no longer has space her laptops drive to store her photos.

The drive she has now is 750GB, so to go to a bigger drive would be expensive, as there isn't many 1.5tb 2.5 inch 9.5mm hdd on the market (as of now, there is only one). Because of that, I am thinking of moving her pictures to a cheap external usb hdd.

As of now, I'm automatically backing up her important folders (My Documents, Pictures, etc.) using Windows 7 default backup software to a network drive.

My problem is that I don't know of a good solution to automatically backup a folder residing on an usb disk.

The usb disk won't be attached to the computer all the time, so I can't just treat it as a normal backup folder. Sometimes the backup would run and the folder would not be there!

Anyone knows any software or methodology to backup folders on external usb hard drives that are not always present?

Thanks